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 A291524 Numbers m such that psi(x) = phi(m) has a solution while sigma(y) = phi(m) has none. 0
 751, 919, 1502, 1838, 2841, 3788, 5682, 6629, 8523, 11251, 11937, 13258, 13669, 14205, 15137, 15152, 15397, 15607, 15916, 16099, 17046, 18940, 19895, 22502, 22728, 23874, 27338, 28410, 30103, 30274, 30304, 30794, 31214, 31832, 32198, 36853, 37880, 39790, 43657 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Values of A069825(k) such that psi(x) = phi(A069825(k)) has a solution for k > 1. Prime terms are 751, 919, 11251, 13669, 15137, ... LINKS EXAMPLE 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. PROG (PARI) is1(n) = my(N=eulerphi(n)); for(k=1, N, if(sigma(k)==N, return(1))); 0; a001615(n) = my(f=factor(n)); prod(i=1, #f~, f[i, 1]^f[i, 2] + f[i, 1]^(f[i, 2]-1)); is2(n) = my(N=eulerphi(n)); for(k=1, N, if(a001615(k)==N, return(1))); 0; isok(n) = !is1(n) && is2(n); \\ after Charles R Greathouse IV at A001615 CROSSREFS Cf. A000010, A000203, A001615, A069825. Sequence in context: A015067 A129036 A290272 * A020391 A252807 A332000 Adjacent sequences:  A291521 A291522 A291523 * A291525 A291526 A291527 KEYWORD nonn AUTHOR Altug Alkan, Aug 25 2017 STATUS approved

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Last modified November 26 21:07 EST 2021. Contains 349344 sequences. (Running on oeis4.)