Number of regions in a complete but borderless regular polygon.

Terms

a(0) =6a(1) =18a(2) =43a(3) =72a(4) =145a(5) =210a(6) =364a(7) =432a(8) =768a(9) =938a(10) =1441a(11) =1680a(12) =2483a(13) =2448a(14) =4010a(15) =4480a(16) =6154a(17) =6798a(18) =9063a(19) =9000a(20) =12901a(21) =13962a(22) =17848a(23) =19152a(24) =24100a(25) =21450a(26) =31869a(27) =33824a(28) =41383a(29) =43758