A111238 - OEIS

%I #13 Apr 27 2024 12:04:23

%S 2,3,9,22,23,24,31,39,40,43,44,47,48,49,50,58,59,66,67,70,76,81,84,88,

%T 89,97,98,100,119,122,123,130,131,138,139,144,145,150,151,152,163,168,

%U 174,178,179,185,197,200,204,207,217,218,221,225,226,227

%N Indices k where A109890(k) is prime.

%H Michael De Vlieger, <a href="/A111238/b111238.txt">Table of n, a(n) for n = 1..10000</a>

%H Michael De Vlieger, <a href="/A111238/a111238.png">Log log scatterplot of A109890(n)</a>, n = 1..10^5, showing primes in red and nonprimes in dark blue.

%t nn = 240; c[_] := False; a[1] = 1; a[2] = 2; s = 3; c[1] = c[2] = True;

%t {2}~Join~Reap[Monitor[Do[k = SelectFirst[Divisors[s], ! c[#] &];

%t   c[k] = True; s += k;

%t If[PrimeQ[k], Sow[n]], {n, 3, nn}], n] ][[-1, 1]] (* _Michael De Vlieger_, Apr 27 2024 *)

%Y Cf. A109890, A111238, A111239, A111240.

%K nonn

%O 1,1

%A _N. J. A. Sloane_, Oct 30 2005