Numbers k such that phi(k-1)+1 divides sigma(k).

Terms

a(0) =3a(1) =5a(2) =17a(3) =26a(4) =171a(5) =257a(6) =265a(7) =1921a(8) =9385a(9) =26665a(10) =65537a(11) =263041a(12) =437761a(13) =1057801a(14) =2038648321a(15) =10866583226a(16) =11453097097a(17) =982923711145