OFFSET
1,2
COMMENTS
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
a(3) = 5 is a member because A000217(5..7) are 15, 21, 28, and 15^21 == 15 (mod 28) where 15 is a triangular number.
MAPLE
tri:= n -> n*(n+1)/2:
istri:= n -> issqr(1+8*n):
select( n -> istri(tri(n) &^ tri(n+1) mod tri(n+2)), [$1..1000]);
MATHEMATICA
Position[Partition[Accumulate[Range[200]], 3, 1], _?(OddQ[Sqrt[1+8*PowerMod[ #[[1]], #[[2]], #[[3]]]]]&), 1, Heads->False]//Flatten (* Harvey P. Dale, Nov 26 2022 *)
PROG
(PARI) tri(n) = n*(n+1)/2; \\ A000217
isok(n) = ispolygonal(lift(Mod(tri(n), tri(n+2))^tri(n+1)), 3); \\ Michel Marcus, Jan 26 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
J. M. Bergot and Robert Israel, Jan 26 2021
STATUS
approved