A039768 - OEIS

A039768

Numbers k such that gcd(phi(k), k-1) = number of divisors of (k-1).

2, 3, 105, 133, 153, 185, 345, 377, 425, 585, 637, 665, 777, 805, 825, 873, 897, 905, 949, 1017, 1090, 1113, 1209, 1225, 1261, 1305, 1309, 1385, 1449, 1525, 1545, 1573, 1645, 1681, 1785, 1813, 1833, 1865, 1885, 1957, 1981, 2009, 2057, 2077, 2105

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OFFSET

1,1

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

EXAMPLE

phi(105)=48, gcd(48,104)=8, 104 is divisible by {1,2,4,8,13,26,52,104}.

MATHEMATICA

aQ[n_] := GCD[EulerPhi[n], n - 1] == DivisorSigma[0, n - 1]; Select[Range[2, 2110], aQ] (* Amiram Eldar, Aug 28 2019 *)

CROSSREFS

Cf. A000005, A000010.

Sequence in context: A041589 A371122 A371229 * A097653 A164926 A129729

Adjacent sequences: A039765 A039766 A039767 * A039769 A039770 A039771

KEYWORD

nonn,easy

AUTHOR

Olivier Gérard

STATUS

approved