cp's OEIS Frontend

A241625 Smallest number m such that the GCD of the x's that satisfy sigma(x)=m is n.

%I A241625 #24 Mar 05 2025 01:49:58
%S A241625 1,3,4,7,6,6187272,8,15,13,196602,8105688,28,14
%N A241625 Smallest number m such that the GCD of the x's that satisfy sigma(x)=m is n.
%C A241625 This sequence is a sequel to A240667.
%C A241625 Some large known terms: a(16)=2031554, a(25)=1355816, a(31)=8880128, a(80)=11532, a(97)=5488.
%C A241625 a(14) > 10^9. - _Michel Marcus_, May 09 2014
%C A241625 a(n) is a multiple of A353783(n). Some further terms: a(15) = 497943732, a(17) = 962949708, a(20) = 612372264, a(48) = 12692888, a(53) = 39887316. - _Max Alekseyev_, Jan 19 2025
%H A241625 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).
%F A241625 For n in A211656, a(n) = sigma(n).
%e A241625 a(2) = 3, because the only x such that sigma(x)=3 is 2.
%e A241625 a(6) = 6187272, because the x's that satisfy sigma(x)=6187272 are [2651676, 2855646] and their GCD is 6.
%o A241625 (PARI) lista() = {lim = 12000000; nn = 100; out = "a241625.txt"; v = vector(lim, i, sigma(i)); w = vector(lim); for (i=1, lim, vi = v[i]; if (vi <= lim, if (w[vi] == 0, w[vi] = i, w[vi] = concat(w[vi], i)););); for (i=1, nn, got = 0; write1(out, i, " "); for (j=1, #w, wj = w[j]; if (gcd(wj) == i, got = 1; write(out, j);break;);); if (! got, write(out, );););}
%o A241625 (PARI) a(n) = my(m=1); while(gcd(invsigma(m)) != n, m++); m; \\ _Michel Marcus_, Jan 16 2025; using _Max Alekseyev_'s invphi.gp
%Y A241625 Cf. A000203, A211656, A240667, A353783, A380303, A380304.
%K A241625 nonn,more,nice
%O A241625 1,2
%A A241625 _Michel Marcus_, Apr 26 2014