1,1

Table of n, a(n) for n=1..49.

a(n) = A130199(n)/2. Multiplying by 2 doesn't change evilness. Hence evil oblong numbers are twice evil triangular numbers.

3 belongs to the sequence because the binary expansion of 3 is 11 - contains an even number of ones.

Select[Table[n(n + 1)/2, {n, 100}], EvenQ[Plus @@ IntegerDigits[ #, 2]] &]

Cf. This sequence is the intersection of evil numbers (A001969) and triangular numbers (A000217).

Sequence in context: A261632 A230364 A214282 * A202269 A151375 A153453

Adjacent sequences: A130197 A130198 A130199 * A130201 A130202 A130203

nonn

Tanya Khovanova, May 16 2007

approved