Number of (w,x,y,z) with all terms in {0,...,n} and |w-x|+|x-y+|y-z|=2n.

Terms

a(0) =1a(1) =6a(2) =16a(3) =26a(4) =42a(5) =58a(6) =82a(7) =106a(8) =140a(9) =174a(10) =220a(11) =266a(12) =326a(13) =386a(14) =462a(15) =538a(16) =632a(17) =726a(18) =840a(19) =954a(20) =1090a(21) =1226a(22) =1386a(23) =1546a(24) =1732a(25) =1918a(26) =2132a(27) =2346a(28) =2590a(29) =2834