47050564domain: NAppears in sequencesNumber of ways of choosing at most n-1 items from a set of size 2*n+1.at n=13A008549a(n) = 2^(n-1) - binomial(n, floor(n/2)).at n=26A093387a(n) = Sum_{k=0..12} binomial(n,k).at n=27A219615a(n) = 2^(n-1) + ((1+(-1)^n)/4)*binomial(n, n/2) - binomial(n, floor(n/2)).at n=27A294175