a(n) = Sum_{k=1..n, gcd(n,k) = 1} binomial(n,k)^2.

Terms

a(0) =1a(1) =4a(2) =18a(3) =32a(4) =250a(5) =72a(6) =3430a(7) =6400a(8) =34506a(9) =29000a(10) =705430a(11) =1254816a(12) =10400598a(13) =8281392a(14) =86567400a(15) =300533760a(16) =2333606218a(17) =2172355848a(18) =35345263798a(19) =68442424000a(20) =332533406646a(21) =554415527952