A276645 Numbers n such that A002088(n) is a triangular number.

0, 1, 4, 5, 9, 19, 148, 209, 404, 652, 763, 1739, 2790, 9551, 11117, 13173, 25641, 170003, 181927, 519900, 3253342, 4816257, 7103608, 8269588, 64261168, 177847445, 294878941, 819667577, 844965349, 2944185248

Offset

1,3

Comments

Corresponding triangular numbers are 0, 1, 6, 10, 28, 120, 6670, 13366, 49770, 129286, 177310, ...

Links

Table of n, a(n) for n=1..30.

Example

9 is a term because A002088(9) = 1+1+2+2+4+2+6+4+6 = 28 = 1+2+3+4+5+6+7.

Prog

(PARI) lista(nn) = {print1(st = 0, ", "); for(n=1, nn, st += eulerphi(n); if (ispolygonal(st, 3), print1(n, ", "))); } \\ Michel Marcus, Sep 09 2016

Crossrefs

Cf. A000217, A002088.

Sequence in context: A280225 A153058 A216225 * A293282 A240926 A041032

Adjacent sequences: A276642 A276643 A276644 * A276646 A276647 A276648

Keyword

nonn,more

Author

Altug Alkan, Sep 09 2016

Extensions

More terms from Michel Marcus, Sep 09 2016

a(25)-a(30) from Lars Blomberg, Oct 15 2016

Status

approved