EXERCISE 4.2.9. Let P be the set of people in a group, with |P| = p. Let C be a set of clubs formed by the people in this group, with |C| = c. Suppose that each club contains exactly g people, and each person is in exactly j clubs. Use two different ways to count the number of pairs (b, h) € P x C such that person b is in club h, and deduce a combinatorial identity.
EXERCISE 4.2.9. Let P be the set of people in a group, with |P| = p. Let C be a set of clubs formed by the people in this group, with |C| = c. Suppose that each club contains exactly g people, and each person is in exactly j clubs. Use two different ways to count the number of pairs (b, h) € P x C such that person b is in club h, and deduce a combinatorial identity.
Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
Problem 1RQ
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EXERCISE 4.2.9. Let P be the set of people in a group, with |P| = p. Let C be a set of clubs
formed by the people in this group, with |C| = c. Suppose that each club contains exactly g
people, and each person is in exactly j clubs. Use two different ways to count the number of pairs (b, h) € P x C such that person b is in club h, and deduce a combinatorial identity.

Transcribed Image Text:in Club h. and deduce a combinational identity,
{ (b.h) = PxC
Let I = SC bin) (PxC (b is in h)
Then
IC PXC
NI P
M₂ I-SC
Persen bis in clubb's
bin) b
(bib) h

Transcribed Image Text:шишу д
Use different ways of counting
the number of pairs
This section is read-
containin
{(b, h) € PxC person bis
club h
Solution
Let
in
Then
I = {(A,^) ( P. C | beit).
we have
Ic PxC
J
pairs (bh)
such that bel
K
We can define maps
St
all pairs
T₁₂: I C
in
7₁: IP
projection
(b, h) b (forget the 27
and
Coordinate
(b) h
"2nd
projection
(forget the 1st
wordinate)
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