a(n) = 2*floor(n*phi) + n - 1, where phi = (1+sqrt(5))/2.

Terms

a(0) =2a(1) =7a(2) =10a(3) =15a(4) =20a(5) =23a(6) =28a(7) =31a(8) =36a(9) =41a(10) =44a(11) =49a(12) =54a(13) =57a(14) =62a(15) =65a(16) =70a(17) =75a(18) =78a(19) =83a(20) =86a(21) =91a(22) =96a(23) =99a(24) =104a(25) =109a(26) =112a(27) =117a(28) =120a(29) =125