A073723 Numbers k such that sigma(k) mod pi(k) = 1.

4, 9, 16, 64, 69, 218, 592, 808, 910, 1921, 1957, 2648, 2860, 7609, 13462, 14953, 16838, 20688, 27050, 44471, 80440, 91860, 122351, 131095, 154606, 166121, 171396, 226831, 257467, 318016, 614626, 726560, 1225277, 1366686, 1465910, 1508284, 1790754, 1816934, 1873100

Offset: 1

Author

Labos Elemer, Aug 05 2002

Keywords

• nonn

Links

• Amiram Eldar, Table of n, a(n) for n = 1..100

Crossrefs

Cf. A000203 (sigma), A000720 (pi), A072548.

Programs

Mathematica

Do[s=Mod[DivisorSigma[1, n], PrimePi[n]]; If[s==1, Print[n]], {n, 1, 1000000}]

PARI

isok(k) = k > 1 && sigma(k) % primepi(k) == 1 \\ Andrew Howroyd, Dec 12 2024

PARI

list(lim) = {my(i = 1, p = 2); forprime(q = 3, lim, for(k = p, q-1, if(sigma(k) % i == 1, print1(k, ", "))); i++; p = q);} \\ Amiram Eldar, Mar 18 2025

Extensions

More terms from Amiram Eldar, Mar 18 2025