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

Terms

a(0) =0a(1) =1a(2) =8a(3) =31a(4) =80a(5) =171a(6) =308a(7) =509a(8) =780a(9) =1137a(10) =1584a(11) =2143a(12) =2812a(13) =3615a(14) =4552a(15) =5645a(16) =6892a(17) =8321a(18) =9924a(19) =11731a(20) =13736a(21) =15967a(22) =18416a(23) =21117a(24) =24056a(25) =27269a(26) =30744a(27) =34515a(28) =38568a(29) =42943