Position of the second n in A026409.

Terms

a(0) =4a(1) =6a(2) =10a(3) =12a(4) =15a(5) =19a(6) =23a(7) =25a(8) =30a(9) =36a(10) =43a(11) =45a(12) =51a(13) =54a(14) =66a(15) =67a(16) =77a(17) =87a(18) =90a(19) =99a(20) =100a(21) =124a(22) =125a(23) =135a(24) =139a(25) =145a(26) =158a(27) =178a(28) =180a(29) =196