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Work in progress / under review

Zsolt Katona: Democracy in Product Design: Consumer Participation and Differentiation Strategies (Revise & Resubmit at the Journal of Marketing Research)
pdf (at SSRN) abstract

Ganesh Iyer and Zsolt Katona: Competing for Attention in Social Communication Markets
(Revise & Resubmit at Management Science) abstract

Zsolt Katona: Competing for Influencers in a Social Network
pdf (at SSRN) abstract

Zsolt Katona, Jonathan Knee, and
Miklos Sarvary:: Agenda Chasing and Contests among News Providers
pdf (at SSRN) abstract

Zsolt Katona and
Yi Zhu: Quality Score that Makes You Invest abstract

Zsolt Katona: Endogenous Homophily in Social Networks
abstract

Zsolt Katona and
Elie Ofek: Quality and Advertising in a Vertically Differentiated Market
abstract
Publications and Accepted Papers

Peter Zubcsek,
Imran Chowdhury and Zsolt Katona: Information Communities:The Network Structure of Communication
(Social Networks, 38 5062, (2014)) pdf (at SSRN) abstract

Ron Berman and Zsolt Katona: The Role of Search Engine Optimization in Search Rankings
(Marketing Science, 32(4) 644651, (2013)) pdf (at SSRN) abstract

Chrysanthos Dellarocas, Zsolt Katona and
William Rand: Media, Aggregators and the Link Economy:
Strategic Hyperlink Formation in Content Networks (Management Science, 59(10) 23602379, (2013))
pdf (at SSRN) abstract

Kaifu Zhang and Zsolt Katona: Contextual Advertising
(Marketing Science 31(6) 980994 (2012)) abstract

Gyula O.H. Katona,
Gyula Y. Katona, and Zsolt Katona: Most Probably Intersecting Families of Subsets,
( Combinatorics, Probability and Computing, 21(March) 219227 (2012))

Zsolt Katona,
Peter Zubcsek and
Miklos Sarvary: Network Effects and Personal Influences:
Diffusion of an Online Social Network
(Journal of Marketing Research 48(3) 425443 (2011))
pdf abstract

Elie Ofek, Zsolt Katona, and
Miklos Sarvary: Bricks and Clicks: The Impact of Product Returns on the Strategies of MultiChannel
Retailers
(Marketing Science 30(1) 4260 (2011)) pdf abstract

Zsolt Katona and
Miklos Sarvary: The Race for Sponsored Links:
Bidding Patterns for Search Advertising
(Marketing Science 29(2) 199215 (2010), lead article, finalist for the Little Award in 2011 and for the Bass award in 2011)
pdf abstract

Joseph Lajos, Zsolt Katona,
Amitava Chattopadhyay, and
Miklos Sarvary: CAM: A Spreading Activation Network Model of Subcategory Construction
(Journal of Consumer Research, Vol 36(1) 122136 (2009) , won the ACR best working paper award)
pdf abstract animation

Zsolt Katona and
Miklos Sarvary: Network Formation and the Structure of the Commercial World Wide Web
(Marketing Science 27(5) 764778 (2009), finalist for the Little Award in 2009 and for the Bass award in 2009 and 2010)
pdf abstract

Zsolt Katona and
Tamás Móri: A new class of scalefree graphs
(Statistics and Probability Letters, Vol 76/15 (2006) 15871593)
pdf abstract

Zsolt Katona: Levels of a scalefree tree (Random Structures and Algorithms, Vol 29/2 (2006) 194207)
pdf abstract

Zsolt Katona: Width of a scalefree tree
(Journal of Applied Probability, Vol 42/3 (2005), 839850)
pdf abstract

Zsolt Katona: 3wise exactly 1intersecting families of sets
(Graphs and Combinatorics, Vol 21/1 (2005), 7176) pdf abstract

Zoltán Füredi and Zsolt Katona: Multiply intersecting families of sets
(Journal of Combinatorial Theory, Series A, Vol 106/2 (2004), 315326) pdf abstract

Zsolt Katona: Intersecting families of sets, no l containing two
common elements (Discrete Mathemathics 226, (2001)
233241) pdf abstract
PhD Dissertation

Buying and Selling Traffic: The Internet as an Advertising Medium
(PhD in Management, INSEAD, 2008) pdf

Random Graph Models
(PhD in Computer Science, Eotvos University, 2007) pdf
Competing for Influencers in a Social Network
Abstract
This paper studies the competition between firms for influencers in a network.
Firms spend effort to convince influencers to recommend their products.
The analysis identifies the offensive and defensive roles of spendgin
on influencers. The value of an influencer only depends on the indegree
distribution of the influence network. Influencers who exclusively cover
a high number of consumers are more valuable to firms than those who mostly
cover consumers also covered by other influencers. Firm profits are
highest when there are many consumers with a very low or with very high
indegree. Consumers with an intermediate level of indegree contribute
negatively to profits and high indegree consumers increase profits when
market competition is not intense. Prices are generally lower when consumers
are covered by many influencers, however, firms are not always worse off with
lower prices.
The nature of consumer response to recommendations makes an important difference.
When first impressions dominate, firm profits for dense networks are higher,
but when recommendations have a cumulative influence profits are reduced as
the network becomes dense.
Agenda Chasing and Contests Among News Providers
Abstract
We model competition among news providers as a contest where each firm chooses
to publish on a topic from a large pool of topics with different prior success
probabilities. If a topic is successful, firms that chose to publish on it share
a fixed reward. We explore how increased competition (as measured by the number
of firms and/or the sharestructure of the reward) and the prior distribution of
topics affect the diversity of published news. We relate our findings to current
trends in news media, characterized by lower barriers to entry and the increased
use of sophisticated technologies to identify successful topics from the large
amount of, both professional and usergenerated content available on the Internet.
We show that the contest nature of competition tends to lead to a broader set of
published media themes with a higher representation for marginal topics. The breadth
of topics increases the more topics follow a ``fattail'' prior distribution and
the more a priori popular topics' success are correlated. It also increases
with the number of competing firms but only if the share of the reward in the contest
dissipates rapidly. We also explore the effect of asymmetry on competition. First,
we assume that some firms have a `brand', i.e. a capability to attract a loyal audience.
We show that branded publishers are more likely to choose topics with high prior success
probabilities, while unbranded publishers tend to choose a priori `unlikely'
topics. Second, we assume that some firms have better forecasting capability for the
topics' success. Surprisingly, in this case, less informed firms choose topics in a
conservative way (i.e. publish topics with the highest prior probabilities). When
many firms reporting on the same topic increases the topic's rate of success, marginal
topics may emerge but only if the contest is not too competitive and competing firms
are neither too few nor too numerous. These findings are related to current trends in
the news media industry.
Democracy in Product Design: Consumer Participation and Differentiation Strategies
Abstract
An increasing number of firms use social media to allow their
customers to vote on new product design. This paper studies the
implications of employing such a democratic product design (DPD).
A linear city model is used with random
locations to capture uncertainty about consumer preferences and to
study strategic forces in monopoly and duopoly settings. The results indicate that a monopolist
will use market research to resolve the demand uncertainty, unless DPD provides a cost advantage.
In duopoly, an asymmetric equilibrium emerges with exactly
only one firm using DPD. Commitment to following consumer votes proves to be a strategic advantage, therefore
at least one firm promises not to deviate from the product design consumers voted for.
One firm would even selfimpose a penalty in order to prevent future deviations. Firm profits
are generally hurt by the use of DPD. The role of strategic consumer participation in voting
is also explored. The results show that a monopolist prefers an intermediate level of participation
cost to ensure an optimal product. In contrast, duopolists are best off when participation costs are
very low or very high, because this allows them to avoid using DPD.
Quality Score that Makes You Invest
Abstract
This paper examines the role of quality score in ad auctions. Many
search engines and other ad auction use a quality score to favor
certain advertisers in the auction. The typical justification for
quality score is the need to favor advertisers with higher
clickthrough rates. This work examines the impact of different
forms of quality scores on advertiser's quality investments. The
results suggests that quality score plays an important role in
providing incentives to advertisers to invest. Often a type of
quality score that is stronger than one that only estimates
clickthrough rates is optimal. Such a quality score not only
increases advertisers qualities, but in turn raises the auctioneer's
revenue. These results are stronger when there are (i) more
advertising slots, (ii) when these slots are similar, and (iii) when
quality investments are expensive.
Contextual Advertising
Abstract
This paper studies the strategic aspects of contextual
advertising. Such advertising entails the display of relevant ads
based on the topic of the content a consumer views and takes
advantage of the possibility that consumers' content browsing
preferences are indicative of their product preferences. The results show that
contextual targeting impacts advertiser profit in two ways: first,
advertising through relevant content topics helps advertisers reach
consumers who have strong preferences for their products. Second,
heterogeneity in consumers' content preferences can be leveraged to
reduce product market competition, even when consumers are
homogeneous in their product preferences. The contextual advertising
intermediary's
incentives to strategically design its content structure and the
targeting precision are governed by the following forces. When
product market competition is high, the intermediary offers
homogeneous content and increases its targeting precision. This
encourages each advertiser to bid for multiple keywords to
preventing its competitors from advertising to the consumers. This
may lead to an asymmetric equilibrium where one advertiser
monopolizes all the advertising spaces to completely preempt
competition. When product market competition is low, the
intermediary offers heterogeneous content but intentionally decrease
its targeting precision. This encourages each advertiser to bid for
multiple advertising spaces in order to reach consumers who prefer
its product.
Media, Aggregators and the Link Economy:
Strategic Hyperlink Formation in Content Networks
Abstract
A key property of the World Wide Web is the possibility for firms to place virtually costless
links to thirdparty content as a substitute or complement to their own content. This ability
to hyperlink has enabled new types of players, such as search engines and content aggregators,
to successfully enter content ecosystems, attracting traffic and revenues by hosting links to the
content of others. This, in turn, has sparked a heated controversy between content producers and
aggregators regarding the legitimacy and social costs/benefits of uninhibited free linking. This
work is the first to model the implications of interrelated and strategic hyperlinking and content
investments. Our results provide a nuanced view of the muchtouted "link economy", highlighting
both the beneficial consequences and the drawbacks of free hyperlinks for content producers and
consumers. We show that content sites can reduce competition and improve profits by forming
links to each other; in such networks one site makes high investments in content and other sites
link to it. Interestingly, competitive dynamics often preclude the formation of link networks, even
in settings where they would improve everyone's profits. Furthermore, such networks improve
economic efficiency only when all members have similar abilities to produce content; otherwise
the less capable nodes can freeride on the content of the more capable nodes, reducing profits
for the capable nodes as well as the average content quality available to consumers. Within these
networks, aggregators have both positive and negative effects. By making it easier for consumers
to access good quality content they increase the appeal of the entire content ecosystem relative
to the alternatives. To the extent that this increases the total traffic flowing into the content
ecosystem, aggregators can help increase the profits of the highest quality content sites. At
the same time, however, the market entry of aggregators takes away some of the revenue that
would otherwise go to pure content sites. Finally, by placing links to only a subset of available
content, aggregators further increase competitive pressure on content sites. Interestingly, this
can increase the likelihood that such sites will then attempt to alleviate the competitive pressure
by forming link networks.
The Race for Sponsored Links:
Bidding Patterns for Search Advertising
Abstract
Web sites invest significant resources in trying to influence their
visibility among online search results. In addition to paying for
sponsored links, they invest in methods known as search engine
optimization (SEO) that improve the ranking of a site
among the search results without improving its quality.
We study the economic incentives of Web sites
to invest in SEO and its implications on search engine and advertiser payoffs.
We find that
the process is equivalent to an allpay auction with noise and
headstarts. Our results show that, under certain
conditions, a positive level of search engine optimization
improves the search engine's ranking quality and thus the satisfaction of
its visitors. In particular, if the quality of sites coincides with
their valuation for visitors then search engine optimization serves
as a mechanism that improves the ranking by correcting measurement
errors. While this benefits consumers and increases traffic to the search engine, sites
participating in search engine optimization could be worse off due to wasteful spending
unless their valuation for traffic is very high.
We also investigate
how search engine optimization affects the revenues from sponsored links.
Surprisingly, we find that in many cases search engine revenues are increased by SEO.
Network Formation and the Structure of the Commercial World Wide Web
Abstract
We model the commercial World Wide Web (WWW) as a
directed graph emerging as the equilibrium of a game in which
utility maximizing Web sites purchase (advertising) inlinks from each
other, while also setting the price of these links.
A key feature of our model is that we consider sites to be heterogeneous
in terms of their ``content", i.e. their inherent value to consumers.
In a world where consumers `surf' on the WWW, sites'
revenues/profits originate from two sources: (i) the sales of
content (products) to consumers, and (ii) the sales of links
(traffic) to other sites. We find that in equilibrium, higher content
sites tend to purchase more advertising links mirroring the DorfmanSteiner rule.
Sites with higher content sell less advertising links and offer such
links at higher prices. As such, there seems to be specialization across sites
in terms of revenue models: high content sites tend to earns revenue from the
sales of content while low content ones from the sales of traffic (advertising),
a tendency that is increasing with the size of the browsing population.
In an extension, we also allow sites to establish (reference) outlinks to each
other beyond the sales of advertising links and find that there is a general
tendency to establish reference link to sites with higher content.
Overall, there is a strong positive correlation between a
site's content and the number of its inlinks. We also explore
network formation in the presence of search engines and
find that the higher the proportion of people using these, the
more sites have an incentive to specialize in certain ``content
areas". Our results have interesting practical implications for
`searchengine optimization', the pricing of Internet advertising as well as the
choice of Internet business models. They also shed light on why successful search
engines (e.g. Google) can use simple heuristics based on inlinks to rank sites
with respect to their content.
Keywords: random graphs, game theory, heterogenous
agents
Network Effects and Personal Influences:
Diffusion of an Online Social Network
Abstract
We study wordofmouth effects on the growth of social networks.
Aggregate diffusion models ignore the possibility that an individual
who is connected to many others in a social network may have a
higher adoption probability (degree effect). Furthermore, the
density of connections in a group of already adopted consumers may
also affect the adoption of individuals connected to this group
(clustering effect). We analyze data from one social network for
which we know the individual connections between every pair of
members. The results support the existence of both degree and
clustering effects. We also show how a linear degree effect at the
individual level can be the underlying determinant of the Bass
model. Furthermore, we present a new methodology to determine the
influential power of every individual. Surprisingly, we find that
highly connected individuals have a lower average influential power
(average influence on a particular person). However, the results
suggest that the large number of connections counterbalances the
small average influential power, such that highly connected
individuals have a higher total influence than their less connected
counterparts. We also find evidence that gender and age affect
influential power.
Keywords: Social Networks, New Product Diffusion
The Role of Search Engine Optimization in Search Rankings
Abstract
Web sites invest significant resources in trying to influence their
visibility in online search results. We study the economic
incentives of Web sites to invest in this process known as search
engine optimization. We focus on methods that improve sites' ranking
among the search results without affecting their quality. We find
that the process is equivalent to an allpay auction with noise and
headstarts. Our results show that in equilibrium, under certain
conditions, some positive level of search engine optimization
improves the search engine's ranking and thus the satisfaction of
its visitors. In particular, if the quality of sites coincides with
their valuation for visitors then search engine optimization serves
as a mechanism that improves the ranking by correcting measurement
errors. While this benefits consumers and search engines, sites
participating in search engine optimization could be worse off
unless their valuation for traffic is very high. We also investigate
how search engine optimization affects sites' investment in content
and find that it can lead to underinvestment as a result of wasteful
spending on search engine optimization.
Keywords: Search engine marketing, Allpay auctions
Competing for Attention in Social Communication Markets
Abstract
We investigate agents' incentives to communicate with their peers in the context
of the emergence of new social media technologies. In particular, we examine the
effects of different types of communication cost structures through which new technologies
change the way agents send and receive messages. These new social networking technologies
make it possible to send the same message to many receivers for the same fixed cost and
for negligible (often zero) marginal cost. This is distinct from traditional wordofmouth
which involves additional marginal cost for each intended recipient. Our results show that
the new social networking technologies may reduce incentives of some individuals to send
messages leading to less diverse communication. As the marginal cost of sending messages
to additional people goes down, senders target more receivers leading to higher levels of
competition for attention between receivers. The intensified competition requires higher effort
on senders' part leading to lower payoffs and, eventually, fewer senders in the population.
Although receivers are targeted by more senders and costs go down and they have access to a
higher variety of messages, the variety of messages sent by the population decreases. We also
study the role of location and horizontal preferences and examine sender differentiation.
Endogenous Homophily in Social Networks
Abstract
It is well known that in most social networks neighbors tend to be
similar to each other. Previous research attributed this phenomenon
to homophily, that is, that individuals like to connect to others
that are similar to them. This paper proposes a network formation
model in which individuals can change their location to become more
similar before linking to each other to benefit more from the
connection. An equilibrium is the outcome describing both the
locations and the network resulting in an endogenously formed
network with endogenous locations. The results show that the
equilibrium network exhibits more homophily than a benchmark with
fixed node locations and the number of links increases as link
formation costs go down. As costs of changing one's location
decrease the network becomes more clustered, less connected and
denser. The results explain how certain individuals become opinion
leaders throughout this process. The paper also provides a framework
that generalizes homophily to examine vertical spaces in addition to
horizontal ones. In a horizontal space the distance between two
nodes determines how much they benefit from the link, whereas in a
vertical space linking to a node with a higher vertical attribute
provides more benefits.
Keywords: Social networks, Homphily, Network formation
Quality and Advertising in a Vertically Differentiated Market
Abstract
We examine firms' quality positions when consumers can only consider
purchasing products that they are informed about through
advertising. Consumers compare the alternatives in their
consideration set and choose the product that maximizes their
utility net of price. Firms choose product quality in a first stage,
advertising strategy in a second stage, and prices in the
last stage. We study two forms of advertising  blanket and
targeted . Under blanket advertising, firms communicate
indiscriminately and a consumer's probability of seeing an ad
depends on the level of ad expenditure. We find that when blanket
advertising is relatively ineffective, i.e., it is costly to ensure
that all consumers are informed, both firms choose a light ad
spending. This allows the firms to be relatively
undifferentiated in qualities, without concern of intense price
competition. When blanket advertising is very effective, the high
quality firm expends heavily on advertising, while its rival
differentiates with a low quality product and expends lightly on
advertising. Interestingly, in a mid range of advertising
effectiveness one firm chooses a high quality product and its rival
positions close by. In this way, the lower quality firm induces its
rival to advertise only lightly to avoid fierce price competition.
Under targeted advertising, firms choose the specific
segment(s) they wish to inform. We identify
conditions such that both firms choose equally high quality
products, but advertise to different segments. This can result in a
middle pocket of unserved consumers, even though consumers with
lower willingness to pay are served.
Keywords: Product Quality, Advertising Strategy, Differentiation, Competition
agents
Information Communities:
The Network Structure of Communication
Abstract
This study puts forward a variable clique overlap model for identifying information communities, or potentially
overlapping subgroups of network actors among whom reinforced independent links ensure efficient
communication. We posit that the intensity of communication between individuals in information communities
is greater than in other areas of the network. Empirical tests show that the variable clique overlap model
is more useful for identifying groups of individuals that have strong internal relationships in closed networks
than those defined by more general models of network closure. These findings extend the scope of network
closure effects proposed by other researchers working with communication networks using social network
methods and approaches, a tradition which emphasizes ties between organizations, groups, individuals, and
the external environment.
Keywords: Communities, Social networks; Communication; Network closure
Bricks and Clicks: The Impact of Product Returns on the Strategies of MultiChannel
Retailers
Abstract
The Internet has increased the flexibility of retailers allowing them to operate an online
arm in addition to their physical stores. While the online channel offers retailers potential
advantages in selling to customer segments that value the conveniences of online shopping,
it also raises new challenges. These include the higher likelihood of costly product returns
when customers' ability to 'touch and feel' the product is important in determining fit. We
study competing retailers operating dual channels ("Bricks and Clicks") and examine how
pricing strategies and the level of physical store assistance change as a result of the additional
Internet outlet. On the supplyside, firms endogenously determine prices and how much to
invest in store characteristics that assist customers in finding matching products (e.g., greater
shelf display capacity, more qualified staff, floor samples). On the demandside, we capture
two relevant sources of customer heterogeneity: (i) retailer preference, and (ii) shopping trip
costs. A central result we obtain is that when differentiation among retailers is not too high,
having an online channel actually increases (costly) investment in store assistance levels
while prices are set higher. We also examine how firms' range of product categories can vary
between the two channels. Our main finding here is that even though it is costless to offer
products online, retailers will sell only a limited assortment over the Internet. In particular,
only "safe" products with a low chance of being returned will be sold online, while retailers'
full range of products will be sold in physical stores.
Keywords: Channels of Distribution, Retailing, Internet Marketing, Pricing, Reverse Logistics, Product
Returns
CAM: A Spreading Activation Network Model of Subcategory Construction
Abstract
A large body of research suggests that people process the entities that they encounter by placing
them into mental categories (Barsalou 1992). Although previous research examines how people access
information in hierarchical category structures, it does not examine how people construct individual
new categories and, in particular, how the locus of these new categories may depend on the structure
of the entire hierarchy. We describe this latter process with a spreading activation model of
hierarchical category structures that we call the Category Activation Model (CAM). In an experiment
and an empirical study, we show that the CAM reliably predicts the probability that a person will
construct a new category at a specific location within a category structure, and we provide evidence
that accessibility is the mechanism that underlies category construction.
Keywords: random graphs, game theory, heterogenous
agents
A new class of scalefree graphs
Abstract
Consider the following modification of the Barab\'asiAlbert random
graph. At every step a new vertex is added to the graph. It is
connected to the old vertices randomly, with probabilities
proportional to the degree of the other vertex, and independently of
each other. We show that the proportion of
vertices of degree $k$ decreases at the rate of $k^{3}$. Furthermore, we prove a strong law of large numbers for the maximum
degree.
Keywords: random graph, scalefree disribution
Levels of a scalefree tree
Abstract
Consider the random graph model of Barabási and Albert, where we add a new vertex in every step and connect it
to some old vertices with probabilities proportional to their degrees. If we connect
it to only one of the old vertices
the graph will be a tree.
These graphs have been shown to have power law degree distributions, the same as
observed in some
large realworld networks. We show that the degree distribution is the same
on every sufficiently high level of the tree.
AMS classification: 05C80, 60C05
Keywords: random graphs, scalefree disribution, width of trees
Width of a scalefree tree
Abstract
Consider the random graph model of Barabási and Albert,
where we add a new vertex in every step and connect it
to some old vertices with probabilities proportional to their degrees.
If we connect it to only one of the old vertices this will be a tree.
These graphs have been shown to have a power law degree distribution,
the same as obsereved in some large realworld networks. We are
interested
in the width of the tree and show that it is W(n) ~ n/sqrt(Pi log n)
and this also holds for a slight generalization of the model with
another constant.
Then we see how this theoretical result can be applied to directory
trees.
AMS classification: 05C80, 60C05
Keywords: random graphs, scalefree disribution, width of trees
3wise exactly 1intersecting families of sets
Abstract
Let f(l,t,n) be the maximal size of a family F a subset of 2^{[n]} such that any l at least 2
sets of F have an exactly
telement intersection.
If l is at least 3, it trivially comes from a paper of Füredi that
the optimal families are trivially intersecting (there is a telement core contained by all the members of the family).
Hence it is easy to determine
f(l,t,n)=[(t/2)(n1)]+1.
Let g(l,t,n) be the maximal size of an lwise exaclty tintersecting family that is not trivially tintersecting.
We give upper and lower bounds which only meet in the following case:
g(3,1,n)=n^{2/3}(1+o(1)).
AMS classification: 05D05; 05B25
Keywords: extremal problems for families
of finite sets; finite projective geometries
Multiply intersecting families of sets (with Zoltán Füredi)
Abstract
Let [n] denote the set {1,2,...,n}, 2^{[n]} the collection of
all
subsets of [n] and F a subset of 2^{[n]} be a family.
The maximum of F is studied if any r subsets have an at least
selement intersection and there are no l subsets containing
t+1 common elements.
We show that F is at most \sum_{i=0}^{ts} \binom{ns}{i}+
\frac{t+\ells}{t+2s}\binom{ns}{t+1s}+\min\{s1, l2\} and
this bound is asymptotically the best possible as n goes to infinty and
t is at least 2s is at least 2, and r,l that are at least 2 are fixed.
AMS classification: 05D05; 05B25
Keywords: extremal problems for families of finite sets
Intersecting families of sets, no l containing two
common elements
Abstract
Let H denote the set {f_{1},f_{2},...,f_{n}},
2^{[n]} the collection of all subsets of H and F
a subset of 2^{[n]} be a family. The maximum of F is
studied if any k subsets have a nonempty intersection and the intersection
of any l distinct subsets (1<_{=}k<l)
is empty. This problem is reduced to a covering problem.
If we have the conditions that any two subsets have a nonempty intersection
and the intersection of any l distinct subsets contains no
two different elements we show that the maximum of F is (l1)n+o(n).
AMS classification: 05D05; 05B25
Keywords: extremal problems for families
of finite sets; finite projective geometries


