this question is equal to whether 8*k-7 is a Square number
A(i) shows  1's position in the sequence
first  A(1)=1
A(2)-A(1)=1
A(3)-A(2)=2
.
.
.
A(n)-A(n-1)=(n-1)
add both side of the “=”
we get An=(n^2-3n+4)/2
       exchange the equation we get
       8*A(n)-7=(2n-3)(2n-3)

but I can't AC```

Edited by author 12.04.2010 15:10

It doesn't seem to work for the case k = 4.