Concentric pentagonal numbers: floor( 5*n^2 / 4 ).

Terms

a(0) =0a(1) =1a(2) =5a(3) =11a(4) =20a(5) =31a(6) =45a(7) =61a(8) =80a(9) =101a(10) =125a(11) =151a(12) =180a(13) =211a(14) =245a(15) =281a(16) =320a(17) =361a(18) =405a(19) =451a(20) =500a(21) =551a(22) =605a(23) =661a(24) =720a(25) =781a(26) =845a(27) =911a(28) =980a(29) =1051