a(n) is the hafnian of the 2n X 2n symmetric matrix defined by M[i, j] = i + j - 1.

A357279

a(n) is the hafnian of the 2n X 2n symmetric matrix defined by M[i, j] = i + j - 1.

Terms

    a(0) =1a(1) =2a(2) =43a(3) =2610a(4) =312081a(5) =61825050a(6) =18318396195

External references