Not of form sigma(k) - phi(k).

Terms

a(0) =1a(1) =3a(2) =4a(3) =6a(4) =8a(5) =9a(6) =12a(7) =13a(8) =17a(9) =19a(10) =21a(11) =25a(12) =29a(13) =31a(14) =37a(15) =41a(16) =43a(17) =45a(18) =46a(19) =49a(20) =51a(21) =53a(22) =55a(23) =57a(24) =58a(25) =61a(26) =65a(27) =69a(28) =70a(29) =71