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A368426
Centered 10-gonal numbers which are sphenic numbers.
1
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
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
KEYWORD
nonn
AUTHOR
Massimo Kofler, Dec 24 2023
STATUS
approved