%I #20 Sep 08 2022 08:46:09
%S 7,8191,8191,131071,524287,524287,2147483647,2147483647,
%T 2305843009213693951,2305843009213693951,2305843009213693951,
%U 2305843009213693951
%N Corresponding values of primes p from A247821 and A247838.
%C Conjecture: all terms are Mersenne primes (A000668).
%C Conjecture: next terms are 2305843009213693951, 2305843009213693951, 2305843009213693951, 2305843009213693951 and 618970019642690137449562111. - _Jaroslav Krizek_, Mar 25 2015
%F a(n) = sigma(sigma(2*A247821(n)-1)) = A000203(A000203(2*A247821(n)-1)) = A051027(2*A247821(n)-1).
%F a(n) = sigma(sigma(A247838(n))) = A000203(A000203(A247838(n))) = A051027(A247838(n)) .
%e a(2) = 8191 because sigma(sigma(2*A247821(2)-1)) = sigma(sigma(A247838(2))) = 8191.
%o (Magma) [SumOfDivisors(SumOfDivisors(n)): n in [A247838(n)]
%Y Cf. A000203, A008438, A247790, A247791, A247820, A247821, A247823, A247954.
%K nonn,more
%O 1,1
%A _Jaroslav Krizek_, Sep 28 2014
%E a(7)-a(8) from _Jaroslav Krizek_, Mar 25 2015
%E a(9)-a(12) from _Giovanni Resta_, Feb 14 2020