Sum of even products minus sum of odd products of different pairs of numbers from 1 to n.

Terms

a(0) =0a(1) =2a(2) =5a(3) =29a(4) =39a(5) =129a(6) =150a(7) =374a(8) =410a(9) =860a(10) =915a(11) =1707a(12) =1785a(13) =3059a(14) =3164a(15) =5084a(16) =5220a(17) =7974a(18) =8145a(19) =11945a(20) =12155a(21) =17237a(22) =17490a(23) =24114a(24) =24414a(25) =32864a(26) =33215a(27) =43799a(28) =44205a(29) =57255