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 A072492 #13 Apr 26 2025 15:26:59
%S A072492 27,35,51,57,65,77,87,93,95,117,119,121,122,123,125,135,143,145,147,
%T A072492 148,155,161,171,177,185,187,189,190,203,205,207,208,209,215,217,219,
%U A072492 220,221,237,245,247,249,250,255,261,267,275,287,289,291,292,297,299
%N A072492 Values of n for which A072491(n)=3.
%C A072492 Define f(1) = 0. For n>=2, let f(n) = n - p where p is the largest prime <= n. A072491(n) = number of iterations of f to reach 0, starting from n.
%C A072492 p+4 is a term if p is a prime but p+2 and p+4 are both composite.
%H A072492 Robert Price, <a href="/A072492/b072492.txt">Table of n, a(n) for n = 1..10000</a>
%e A072492 27 is a term as f(27)=27-23=4, f(4)=4-3=1 and f(1) = 0. (3 steps.)
%t A072492 f[1]=0; f[n_] := n-Prime[PrimePi[n]]; a72491[n_] := Module[{k, x}, For[k=0; x=n, x>0, k++; x=f[x], Null]; k]; Select[Range[300], a72491[ # ]==3&]
%Y A072492 Cf. A072491.
%K A072492 nonn,easy
%O A072492 1,1
%A A072492 _Amarnath Murthy_, Jul 14 2002
%E A072492 Edited by _Dean Hickerson_, Nov 26 2002