a(n) = (2^n)!/4^n, with a(1)=1, a(2)=2.A046856a(n) = (2^n)!/4^n, with a(1)=1, a(2)=2.Terms a(0) =1a(1) =2a(2) =630a(3) =81729648000External referencesoeis: A046856