Fourth spoke of a hexagonal spiral.

Terms

a(0) =1a(1) =5a(2) =15a(3) =31a(4) =53a(5) =81a(6) =115a(7) =155a(8) =201a(9) =253a(10) =311a(11) =375a(12) =445a(13) =521a(14) =603a(15) =691a(16) =785a(17) =885a(18) =991a(19) =1103a(20) =1221a(21) =1345a(22) =1475a(23) =1611a(24) =1753a(25) =1901a(26) =2055a(27) =2215a(28) =2381a(29) =2553