Pentagonal numbers: a(n) = n*(3*n-1)/2.

Terms

a(0) =0a(1) =1a(2) =5a(3) =12a(4) =22a(5) =35a(6) =51a(7) =70a(8) =92a(9) =117a(10) =145a(11) =176a(12) =210a(13) =247a(14) =287a(15) =330a(16) =376a(17) =425a(18) =477a(19) =532a(20) =590a(21) =651a(22) =715a(23) =782a(24) =852a(25) =925a(26) =1001a(27) =1080a(28) =1162a(29) =1247