Gaussian binomial coefficient [n, 2] for q = 2.

Terms

a(0) =0a(1) =0a(2) =1a(3) =7a(4) =35a(5) =155a(6) =651a(7) =2667a(8) =10795a(9) =43435a(10) =174251a(11) =698027a(12) =2794155a(13) =11180715a(14) =44731051a(15) =178940587a(16) =715795115a(17) =2863245995a(18) =11453115051a(19) =45812722347a(20) =183251413675a(21) =733006703275