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 A050988 #43 Feb 16 2025 08:32:41
%S A050988 1,42,1008,18144,272160,3592512,43110144,480370176,5043886848,
%T A050988 50438868480,484213137408,4489976365056,40409787285504,
%U A050988 354362750042112,3037395000360960,25514118003032064,210491473525014528,1708695490967764992,13669563927742119936,107917609955858841600
%N A050988 6-idempotent numbers.
%C A050988 Number of n-permutations of 7 objects: s, t, u, v, z, x, y with repetition allowed, containing exactly six u's. - _Zerinvary Lajos_, Jun 16 2008
%H A050988 Vincenzo Librandi, <a href="/A050988/b050988.txt">Table of n, a(n) for n = 6..400</a>
%H A050988 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/IdempotentNumber.html">Idempotent Number</a>.
%F A050988 a(n) = binomial(n,6)*6^(n-6).
%F A050988 G.f.: x^6/(1-6*x)^7. - _Zerinvary Lajos_, Aug 09 2008
%F A050988 From _Amiram Eldar_, Apr 17 2022: (Start)
%F A050988 Sum_{n>=6} 1/a(n) = 102561/5 - 112500*log(6/5).
%F A050988 Sum_{n>=6} (-1)^n/a(n) = 605052*log(7/6) - 466341/5. (End)
%p A050988 seq(binomial(n, 6)*6^(n-6), n=6..32); # _Zerinvary Lajos_, Jun 16 2008
%p A050988 A050988:=x^6/(1-6*x)^7; series(A050988,x,32); # _Zerinvary Lajos_, Aug 09 2008
%t A050988 a[n_] := Binomial[n, 6]*6^(n - 6); Array[a, 20, 6] (* _Amiram Eldar_, Apr 17 2022 *)
%Y A050988 Cf. A000027, A001788, A036216, A040075, A050982, A050989.
%K A050988 nonn,easy
%O A050988 6,2
%A A050988 _Eric W. Weisstein_