A068474 Numbers k such that phi((prime(k)-1)/2) = sigma(k).

19, 47, 77, 79, 115, 177, 191, 267, 280, 297, 340, 398, 406, 446, 470, 639, 839, 917, 1240, 1655, 1713, 1778, 1836, 2140, 2157, 4715, 5110, 5269, 6585, 7089, 7965, 8517, 8844, 9785, 10281, 12586, 15279, 16100, 16468, 16732, 17724, 18145, 18256, 20728

Offset

1,1

Links

Amiram Eldar, Table of n, a(n) for n = 1..300 (terms below 10^9, terms 1..130 from G. C. Greubel)

Mathematica

Select[Range[2, 3000], EulerPhi[(Prime[#]-1)/2]==DivisorSigma[1, #]&] (* G. C. Greubel, Oct 13 2018 *)

Prog

(PARI) for(n=2, 45000, if(eulerphi((prime(n)-1)/2)==sigma(n), print1(n, ", ")))

Crossrefs

Cf. A000010, A000203.

Sequence in context: A166148 A059657 A048970 * A201314 A289728 A141973

Adjacent sequences: A068471 A068472 A068473 * A068475 A068476 A068477

Keyword

easy,nonn

Author

Benoit Cloitre, Mar 10 2002

Status

approved