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1, 3, 5, 7, 8, 9, 10, 11, 13, 19, 20, 21, 23, 25, 29, 30, 31, 33, 34, 35, 36, 37, 43, 44, 45, 49, 50, 55, 56, 58, 59, 61, 62, 63, 66, 68, 70, 71, 72, 74, 75, 77, 79, 80, 81, 83, 85, 91, 93, 94, 103, 104, 106, 108, 115, 117, 118, 119, 124, 125, 127, 128, 131, 138, 139, 143, 144, 153, 154, 155, 157
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OFFSET
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1,2
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COMMENTS
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LINKS
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EXAMPLE
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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.
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MAPLE
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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]);
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MATHEMATICA
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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 *)
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PROG
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(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
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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