1,3

a(n) = A037952(n) - 1. Proof by Ira Gessel: Write down the number of such subsets with k elements <= (n-1)/2 as a product of two binomial coefficients, then evaluate the sum using Vandermonde's theorem.

Sequence in context: A113501 A101067 A056645 this_sequence A094557 A026307 A139816

Adjacent sequences: A047168 A047169 A047170 this_sequence A047172 A047173 A047174

nonn

Clark Kimberling (ck6(AT)evansville.edu)