a(n) is the number of trailing zeros in (2^n)!.

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =3a(5) =7a(6) =14a(7) =31a(8) =63a(9) =126a(10) =253a(11) =509a(12) =1021a(13) =2045a(14) =4094a(15) =8189a(16) =16380a(17) =32763a(18) =65531a(19) =131067a(20) =262140a(21) =524285a(22) =1048571a(23) =2097146a(24) =4194297a(25) =8388603a(26) =16777208a(27) =33554424a(28) =67108858a(29) =134217720