

A137281


Numbers k such that T(k) is not divisible by T(i), 1 < i < k, where T(k) = kth triangular number A000217(k).


5



2, 4, 7, 10, 13, 16, 22, 25, 28, 31, 34, 37, 43, 46, 49, 52, 58, 61, 67, 70, 73, 76, 82, 85, 88, 94, 97, 103, 106, 118, 121, 127, 130, 133, 136, 142, 145, 148, 151, 157, 163, 166, 169, 172, 178, 187, 190, 193, 196, 202, 205, 208, 211, 214, 217, 226, 229, 232, 238, 241
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OFFSET

1,1


COMMENTS

All terms except 2 are congruent to 1 (mod 3). This is required for 3 not to be a divisor of T(n).  Franklin T. AdamsWatters, Dec 10 2019


LINKS



FORMULA



EXAMPLE

T(4)=10 is not divisible by lesser T's 3, 6;
T(7)=28 is not divisible by lesser T's 3, 6, 10, 15, 21.


MATHEMATICA

nn = 241; tri = Table[n*(n+1)/2, {n, nn}]; Select[Range[2, nn], ! MemberQ[Mod[tri[[#]], Take[tri, {2, #  1}]], 0] &] (* T. D. Noe, Apr 12 2011 *)


CROSSREFS



KEYWORD

nonn


AUTHOR



STATUS

approved



