A368426 Centered 10-gonal numbers which are sphenic numbers.

20801, 22781, 37411, 47531, 55651, 75031, 80011, 100111, 120901, 133661, 161101, 177661, 191101, 199001, 201001, 230051, 236531, 240901, 245311, 263351, 279661, 289201, 313751, 323851, 326401, 368561, 376751, 436601, 439561, 445511, 472781, 475861, 488281, 507211, 539561

Offset

1,1

Links

Robert Israel, Table of n, a(n) for n = 1..5080

Example

A062786(65) = 20801 = 5 * 65 * (65-1) + 1 = 11 * 31 * 61.
A062786(68) = 22781 = 5 * 68 * (68-1) + 1 = 11 * 19 * 109.

Maple

isc10:= proc(n) issqr(20*n+5) end proc:
P:= select(isprime, [seq(seq(10*i+j, j=[-1, 1]), i=1..1000)]): nP:= nops(P):
M:= P[1]*P[2]*P[-1]:
A:= NULL:
for i from 1 to nP-2 while P[i]^3 < M do
  for j from i+1 to nP-1 while P[i]*P[j]^2 < M do
    for k from j+1 to nP do
      q:= P[i]*P[j]*P[k];
      if q > M then break fi;
      if isc10(q) then A:= A, q fi
od od od:
sort([A]); # Robert Israel, Dec 24 2023

Mathematica

Select[Table[5*n*(n+1) + 1, {n, 1, 330}], FactorInteger[#][[;; , 2]] == {1, 1, 1} &] (* Amiram Eldar, Dec 24 2023 *)

Crossrefs

Intersection of A007304 and A062786.

Sequence in context: A104903 A235952 A207794 * A256840 A031813 A043654

Adjacent sequences: A368423 A368424 A368425 * A368427 A368428 A368429

Keyword

nonn

Author

Massimo Kofler, Dec 24 2023

Status

approved