Problems & Puzzles:
and sum of repunits
Carlos Rivera asks this:
R(n) is a "Repunit" composed of n digits "1" then
= R(1)+R(2)+…R(n) is prime for n=2497 and 3301.
Q. Find more n values such that S(R(n)) is prime?
*Perhaps helps this closed form for S(R(n))=(10^(n+1)-9*n-10)/81.
All these contributions came along the week 16-22 Nov 2019 from Oscar
Volpatti, Simon Cavegn, Gennady Gusev
I checked the repunit sums with length up to 75001 digits, finding
no further primes.
Searched up to n=60192.
Did not find any.