This is a subsequence of A077374.
Except for the first term, all known terms of this sequence are divisible by 15. Is there a number n > 1 such that gcd(a(n),3)=1 or gcd(a(n),5)=1?
a(6) > 10^13.  Giovanni Resta, Mar 29 2013
Also, a subsequence of A141548.  M. F. Hasler, Apr 12 2015
The terms a(3) through a(5) are of the form a(k)*p*q, but I have proved that there is no other term of this form with k <= 5.  M. F. Hasler, Apr 13 2015
The terms are also of the form a(n) = 2*p(n) + 1, with primes p(n) = 3, 7, 157, 577, 407817217. All but the last one are such that 2*p(n)  1 = a(n)  2 is again prime.  M. F. Hasler, Nov 27 2016
Terms a(2..5) satisfy 2*a(n)  nextprime(sigma(a(n))) = (1)^n, see also A067795.  M. F. Hasler, Feb 14 2017
