A085347 Least number x so that number of primes not exceeding sigma(x) equals n.

0, 2, 5, 4, 6, 8, 10, 19, 12, 29, 16, 18, 20, 43, 33, 28, 24, 32, 67, 30, 45, 57, 44, 36, 52, 101, 63, 85, 91, 48, 64, 86, 137, 66, 149, 99, 106, 163, 60, 129, 88, 80, 105, 72, 197, 104, 100, 84, 147, 229, 90, 114, 112, 96, 171, 148, 136, 201, 108, 281, 138, 152, 172

Offset: 1

Author

Labos Elemer, Jul 10 2003

Keywords

• nonn

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..500

Crossrefs

Cf. A000720, A000010, A000203, A070803, A070804, A085341-A085347.

Programs

Mathematica

{0}~Join~Table[x = 1; While[PrimePi@ DivisorSigma[1, x] != n, x++]; x, {n, 2, 63}] (* Michael De Vlieger, Feb 05 2017 *)

PARI

a(n) = if (n==1, 0, x = 1; while(primepi(sigma(x))!= n,  x++); x); \\ Michel Marcus, Dec 11 2013

Formula

a(n) = Min{x; A070803(x)=n};