Floor( 7*n^2/2 ).

Terms

a(0) =0a(1) =3a(2) =14a(3) =31a(4) =56a(5) =87a(6) =126a(7) =171a(8) =224a(9) =283a(10) =350a(11) =423a(12) =504a(13) =591a(14) =686a(15) =787a(16) =896a(17) =1011a(18) =1134a(19) =1263a(20) =1400a(21) =1543a(22) =1694a(23) =1851a(24) =2016a(25) =2187a(26) =2366a(27) =2551a(28) =2744a(29) =2943