1,3

Table of n, a(n) for n=1..38.

66 is in the sequence because (1) it is a triangular number and (2) the sum of its digits 6+6=12 is a pentagonal number.

(PARI) isok(n) = ispolygonal(n, 3) && ispolygonal(sumdigits(n), 5); \\ Michel Marcus, Feb 26 2014

Cf. A000217, A000326.

Sequence in context: A163615 A232062 A229003 * A250790 A231886 A278090

Adjacent sequences: A117302 A117303 A117304 * A117306 A117307 A117308

base,nonn

Luc Stevens (lms022(AT)yahoo.com), Apr 26 2006

approved