Sequence 1 / 2, 1 / 4, - 5 / 8, 13 / 16, - 29 / 32, 61 / 64

Sequence 1 / 2, 1 / 4, - 5 / 8, 13 / 16, - 29 / 32, 61 / 64

a=(2^n-3)/2^n*(-1)^n (n>1)
a=1/2 (n=1)