Numbers n such that 2n-1 and sigma(n) are both primes.

Terms

a(0) =2a(1) =4a(2) =9a(3) =16a(4) =64a(5) =289a(6) =1681a(7) =2401a(8) =3481a(9) =4096a(10) =15625a(11) =65536a(12) =85849a(13) =262144a(14) =491401a(15) =531441a(16) =552049a(17) =683929a(18) =703921a(19) =734449a(20) =1352569a(21) =1885129a(22) =3411409a(23) =3892729a(24) =5470921a(25) =7091569a(26) =7778521a(27) =9247681a(28) =10374841a(29) =12652249