1,3

Equivalently, a(n) is the number of fractions x/y, with 1<=x,y<=n, that reduce to (even)/(odd).

Table of n, a(n) for n=1..57.

a(n)=(n^2-A135013)/2

Sequence in context: A191175 A191125 A001225 * A134640 A216094 A184857

Adjacent sequences: A156998 A156999 A157000 * A157002 A157003 A157004

nonn

Adam McDougall (mcdougall.adam(AT)gmail.com), Feb 20 2009

approved