A352990 Numbers k such that the k-th triangular number == 1 mod the integer log of k.

2, 6, 12, 21, 33, 45, 52, 63, 136, 162, 201, 205, 208, 225, 245, 253, 301, 304, 344, 441, 494, 531, 550, 637, 697, 720, 742, 806, 901, 910, 918, 1078, 1233, 1242, 1274, 1333, 1376, 1540, 1566, 1573, 1625, 1680, 1695, 1792, 1863, 1909, 2025, 2041, 2107, 2295, 2466, 2497, 2774, 2896, 2926, 2965

Offset

1,1

Comments

Numbers k such that A000217(k) == 1 (mod A001414(k)).

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Example

a(3) = 12 = 2*2*3 is a term because 12*13/2 = 78 == 1 (mod 2+2+3 = 7).

Maple

filter:= proc(n) local t; (n*(n+1)/2) mod add(t[1]*t[2], t=ifactors(n)[2]) = 1 end proc:
select(filter, [$2..3000]);

Mathematica

Select[Range[3000], Mod[#*(# + 1)/2, Plus @@ Times @@@ FactorInteger[#]] == 1 &] (* Amiram Eldar, Apr 14 2022 *)

Crossrefs

Cf. A000217, A001414, A352989.

Sequence in context: A323291 A161203 A292768 * A184637 A022450 A011892

Adjacent sequences: A352987 A352988 A352989 * A352991 A352992 A352993

Keyword

nonn

Author

J. M. Bergot and Robert Israel, Apr 13 2022

Status

approved