A111238 Indices k where A109890(k) is prime.

2, 3, 9, 22, 23, 24, 31, 39, 40, 43, 44, 47, 48, 49, 50, 58, 59, 66, 67, 70, 76, 81, 84, 88, 89, 97, 98, 100, 119, 122, 123, 130, 131, 138, 139, 144, 145, 150, 151, 152, 163, 168, 174, 178, 179, 185, 197, 200, 204, 207, 217, 218, 221, 225, 226, 227

Offset: 1

Author

N. J. A. Sloane, Oct 30 2005

Keywords

• nonn

Links

• Michael De Vlieger, Table of n, a(n) for n = 1..10000
• Michael De Vlieger, Log log scatterplot of A109890(n), n = 1..10^5, showing primes in red and nonprimes in dark blue.

Crossrefs

Cf. A109890, A111238, A111239, A111240.

Programs

Mathematica

nn = 240; c[_] := False; a[1] = 1; a[2] = 2; s = 3; c[1] = c[2] = True;
{2}~Join~Reap[Monitor[Do[k = SelectFirst[Divisors[s], ! c[#] &];
  c[k] = True; s += k;
If[PrimeQ[k], Sow[n]], {n, 3, nn}], n] ][[-1, 1]] (* Michael De Vlieger, Apr 27 2024 *)