a(n) is the integer part of the area of a regular n-gon whose side lengths are n.

Terms

a(0) =3a(1) =16a(2) =43a(3) =93a(4) =178a(5) =309a(6) =500a(7) =769a(8) =1133a(9) =1612a(10) =2228a(11) =3005a(12) =3969a(13) =5147a(14) =6570a(15) =8268a(16) =10275a(17) =12627a(18) =15360a(19) =18514a(20) =22130a(21) =26250a(22) =30921a(23) =36187a(24) =42099a(25) =48707a(26) =56063a(27) =64221a(28) =73239a(29) =83174