Convolution of nonzero heptagonal numbers (A000566) with themselves.

Terms

a(0) =1a(1) =14a(2) =85a(3) =320a(4) =910a(5) =2156a(6) =4494a(7) =8520a(8) =15015a(9) =24970a(10) =39611a(11) =60424a(12) =89180a(13) =127960a(14) =179180a(15) =245616a(16) =330429a(17) =437190a(18) =569905a(19) =733040a(20) =931546a(21) =1170884a(22) =1457050a(23) =1796600a(24) =2196675a(25) =2665026a(26) =3210039a(27) =3840760a(28) =4566920a(29) =5398960