Number of (w,x,y,z) with all terms in {0,...,n}, w even, x even, and w+x=y+z.

Terms

a(0) =1a(1) =1a(2) =8a(3) =10a(4) =29a(5) =35a(6) =72a(7) =84a(8) =145a(9) =165a(10) =256a(11) =286a(12) =413a(13) =455a(14) =624a(15) =680a(16) =897a(17) =969a(18) =1240a(19) =1330a(20) =1661a(21) =1771a(22) =2168a(23) =2300a(24) =2769a(25) =2925a(26) =3472a(27) =3654a(28) =4285a(29) =4495