A309203 Numbers k such that A111273(k-1) = k.

3, 5, 7, 11, 13, 17, 19, 23, 27, 29, 31, 35, 37, 39, 41, 43, 45, 47, 53, 55, 59, 61, 63, 67, 71, 73, 77, 79, 83, 89, 95, 97, 99, 101, 103, 107, 109, 113, 115, 119, 125, 127, 131, 135, 137, 139, 141, 147, 149, 151, 155, 157, 163, 167, 171, 173, 179, 181, 191, 193, 197, 199, 207, 211, 215, 219

Offset: 1

Author

N. J. A. Sloane, Jul 27 2019

Keywords

• nonn

Comments

All terms are odd; includes all odd primes.

The odd terms that are not primes are 27, 35, 39, 45, 55, 63, 77, 95, 99, 115, 119, 125, 135, 141, 147, 155, 171, .... What are these numbers?

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A111273.

Programs

Maple

S:= {}:
R:= NULL: count:= 0:
for n from 1 while count < 200 do
  v:= min(numtheory:-divisors(n*(n+1)/2) minus S);
  S:= S union {v};
  if v = n+1 then R:= R, n+1; count:= count+1 fi;
od:
R; # Robert Israel, Jul 30 2019