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

1,1

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

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

Crossrefs

Cf. A111273.

Sequence in context: A122089 A109860 A100392 * A176847 A067019 A132106

Adjacent sequences: A309200 A309201 A309202 * A309204 A309205 A309206

Keyword

nonn

Author

N. J. A. Sloane, Jul 27 2019

Status

approved