This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A377721 #6 Nov 05 2024 12:19:20 %S A377721 1,3,4,5,6,8,9,10,11,12,13,14,15,16,18,19,20,21,22,23,24,25,26,27,28, %T A377721 29,30,31,32,33,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52, %U A377721 53,54,55,56,57,58,59,61,62,63,64,65,66,67,68,69,70,71,72 %N A377721 Numbers that are not of the form k(k+1)(2k+1)/6 + k. %C A377721 Complement of A145066. Numbers that are not of the form A000330(k)+k. %F A377721 a(n) = n+m if n > m(m+1)(2m+1)/6 and a(n) = n+m-1 otherwise where m = floor((3n)^(1/3)). %F A377721 a(n) = A074279(n)+n-1. %o A377721 (Python) %o A377721 from sympy import integer_nthroot %o A377721 def A377721(n): return n+(m:=integer_nthroot(3*n,3)[0])-(6*n<=m*(m+1)*((m<<1)+1)) %Y A377721 Cf. A000330, A074279, A145066. %K A377721 nonn %O A377721 1,2 %A A377721 _Chai Wah Wu_, Nov 04 2024