Number of (w,x,y,z) with all terms in {1,...,n} and 2|w-x|=|x-y|+|y-z|.

Terms

a(0) =0a(1) =1a(2) =4a(3) =17a(4) =42a(5) =85a(6) =142a(7) =235a(8) =346a(9) =495a(10) =680a(11) =911a(12) =1172a(13) =1505a(14) =1872a(15) =2305a(16) =2798a(17) =3365a(18) =3978a(19) =4699a(20) =5470a(21) =6335a(22) =7284a(23) =8335a(24) =9448a(25) =10705a(26) =12028a(27) =13473a(28) =15026a(29) =16709