cp's OEIS Frontend

A291524 Numbers m such that psi(x) = phi(m) has a solution while sigma(y) = phi(m) has none.

%I A291524 #25 Sep 02 2017 13:44:30
%S A291524 751,919,1502,1838,2841,3788,5682,6629,8523,11251,11937,13258,13669,
%T A291524 14205,15137,15152,15397,15607,15916,16099,17046,18940,19895,22502,
%U A291524 22728,23874,27338,28410,30103,30274,30304,30794,31214,31832,32198,36853,37880,39790,43657
%N A291524 Numbers m such that psi(x) = phi(m) has a solution while sigma(y) = phi(m) has none.
%C A291524 Values of A069825(k) such that psi(x) = phi(A069825(k)) has a solution for k > 1.
%C A291524 Prime terms are 751, 919, 11251, 13669, 15137, ...
%e A291524 751 is a term because psi(x) = phi(751) = 750 has a solution that is x = 625 while there is no solution for sigma(y) = phi(751) = 750.
%o A291524 (PARI) is1(n) = my(N=eulerphi(n)); for(k=1, N, if(sigma(k)==N, return(1))); 0;
%o A291524 a001615(n) = my(f=factor(n)); prod(i=1, #f~, f[i, 1]^f[i, 2] + f[i, 1]^(f[i, 2]-1));
%o A291524 is2(n) = my(N=eulerphi(n)); for(k=1, N, if(a001615(k)==N, return(1))); 0;
%o A291524 isok(n) = !is1(n) && is2(n); \\ after _Charles R Greathouse IV_ at A001615
%Y A291524 Cf. A000010, A000203, A001615, A069825.
%K A291524 nonn
%O A291524 1,1
%A A291524 _Altug Alkan_, Aug 25 2017