Sum over all permutations beginning and ending with ascents, and without double ascents on n elements and each permutation contributes 2 to the power of the number of double descents.
A227918
Sum over all permutations beginning and ending with ascents, and without double ascents on n elements and each permutation contributes 2 to the power of the number of double descents.
Terms
- a(0) =1a(1) =0a(2) =5a(3) =22a(4) =137a(5) =956a(6) =7653a(7) =68874a(8) =688745a(9) =7576192a(10) =90914309a(11) =1181886014a(12) =16546404201a(13) =248196063012
External references
- oeis: A227918