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 A363627 #18 Jun 17 2023 14:22:48 %S A363627 1,2,3,2,5,3,7,4,3,5,11,8,13,7,5,8,17,12,19,10,7,11,23,18,5,13,9,14, %T A363627 29,20,31,16,11,17,7,27,37,19,13,32,41,36,43,22,27,23,47,36,7,25,17, %U A363627 26,53,36,11,32 %N A363627 a(n) = greatest product < n of some subset of the divisors of n, or if n is in A008578 then a(n) = n. %C A363627 a(n) = n <=> n in A008578. %C A363627 For composite n, a(n) < n < A363501(n) and where both bounds are products of divisors of n and as tight as possible. %e A363627 n = 4; divisors: [1,2,4]; subsets: [[], [1], [2], [4], [1, 2], [1, 4], [2, 4], [1, 2, 4]]; products: [1, 1, 2, 4, 2, 4, 8, 8]; the maximal product that is lesser than 4 is 2, so a(4) = 2. %t A363627 If[PrimeQ@n || n == 1, n, %t A363627 Last@Select[Union[Times @@@ Subsets[Divisors@n]], # < n &]]; %o A363627 (PARI) a(n) = my(d=divisors(n), nb = #d, m=1); forsubset(nb, s, my(p=vecprod(vector(#s, k, d[s[k]]))); if (p<n, m=max(m, p))); if (m>1, m, n); \\ _Michel Marcus_, Jun 17 2023 %Y A363627 Cf. A008578, A363501. %K A363627 nonn %O A363627 1,2 %A A363627 _Denis Ivanov_, Jun 12 2023