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 A072681 #24 Apr 17 2025 09:55:37
%S A072681 0,0,1,0,1,0,3,4,3,0,1,0,3,4,3,0,1,0,3,4,3,0,5,8,9,8,5,0,1,0,5,8,9,8,
%T A072681 5,0,3,4,3,0,1,0,3,4,3,0,5,8,9,8,5,0,5,8,9,8,5,0,1,0,5,8,9,8,5,0,3,4,
%U A072681 3,0,1,0,5,8,9,8,5,0,3,4,3,0,5,8,9,8,5,0,7,12,15,16,15,12,7,0,3,4,3,0,1,0
%N A072681 a(n) = (n - A007917(n)) * (A007918(n) - n).
%C A072681 a(n)=0 iff n is prime.
%C A072681 Local maxima occur at interprimes: a(A024675(n)) = A074927(n+1). - _Reinhard Zumkeller_, Mar 04 2009
%C A072681 Expanding upon the maxima comment, repetitive subset triplets (like 3,4,3) of form (k,k+1,k) occur when the middle value is a square. - _Bill McEachen_, Apr 14 2025
%H A072681 Reinhard Zumkeller, <a href="/A072681/b072681.txt">Table of n, a(n) for n = 2..10000</a>
%F A072681 a(n) = A064722(n) * A007920(n).
%F A072681 a(n) = A064722(n) * (A072680(n) - A064722(n)).
%t A072681 a[n_] := (n - NextPrime[n+1, -1])*(NextPrime[n] - n); Table[a[n], {n, 2, 103}] (* _Jean-François Alcover_, Jun 14 2013 *)
%Y A072681 Cf. A007917, A007918, A007920, A024675, A064722, A072680, A074927.
%K A072681 nonn,nice,look
%O A072681 2,7
%A A072681 _Reinhard Zumkeller_, Jul 01 2002