A091189 -id:A091189 - cp's OEIS frontend

A056660 Numbers k such that 20*R_k + 1 is a prime, where R_k = 11...1 is the repunit (A002275) of length k.

Original entry on oeis.org

3, 17, 99, 120, 243, 545, 630, 1493, 2565, 8087

Offset: 1

Robert G. Wilson v, Aug 09 2000

nonn

Also numbers k such that (2*10^(k+1)-11)/9 is a prime.

Makoto Kamada, Prime numbers of the form 22...221.
Index entries for primes involving repunits

Cf. A002275, A084832, A091189 (corresponding primes).

Do[ If[ PrimeQ[ 20*(10^n - 1)/9 + 1 ], Print[n]], {n, 7000}]

a(n) = A084832(n) - 1. - Robert Price, Jan 30 2015

a(8) and a(9) from Rick L. Shepherd, Feb 22 2004

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

A096843 Primes of form repdigit - 1. Primes whose sum of divisors is a decimal repdigit.

Original entry on oeis.org

2, 3, 5, 7, 43, 443, 887, 2221, 8887, 444443, 888887, 444444443, 888888887, 444444444443, 888888888887, 222222222222222221, 444444444444444444444444444443, 44444444444444444444444444444443

Offset: 1

Labos Elemer, Jul 15 2004

Union numbers 2, 5 and sequences A093171, A093163 and A091189.

Corresponding values of sigma(a(n)) are in A028987. - Jaroslav Krizek, Mar 19 2013

			n=43: sigma(43)=44;

Giovanni Resta, Table of n, a(n) for n = 1..33 (terms < 10^1000)

Cf. A096503-A096508, A096841-A096846, A000203.

Missing a(1)=2 and a(3)=5 added by Jaroslav Krizek, Mar 19 2013

cp's OEIS Frontend

A056660 Numbers k such that 20*R_k + 1 is a prime, where R_k = 11...1 is the repunit (A002275) of length k.

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