tiny-tim,
for even power, (a + b)2n,
let n=1,
1 2 1
we have gcd(2ab, a+b)
here gcd=1/2
let n=2,
1 4 6 4 1
we have gcd(2ab(2a2+3ab+2b2), a+b)
here gcd=1/2
let n=3,
1 6 15 20 15 6 1
we have gcd(ab(6a4+15a3b+20a2b2+15ab3+6b4), a+b)
here gcd=1
let n=4,
1 8 28 56 70 56 28...