- #1
geoffrey159
- 535
- 72
Homework Statement
Find a recursive relation on the number of partitions ##P_n## for a set ##S_n## of cardinal ##n##. ##P_0 = 1## is given.
Homework Equations
The Attempt at a Solution
A partition of ##S_{n+1}## is given by the choice of a non-empty ##k##-block ##A_k## of ##S_{n+1}## and a partition of ##S_{n+1} - A_k## which has cardinality ##n+1-k##, for all ##k \in [[ 1...n+1 ]]##. I find :
##P_{n+1} = \sum_{k = 1}^{n+1} {n+1\choose k} P_{n+1-k} = \sum_{k = 1}^{n+1} {n+1\choose n+1-k} P_{n+1-k} = \sum_{s = 0}^{n} {n+1\choose s} P_s ##
But this formula is wrong and I don't understand why. What did I miss ?