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A271585 Numbers k such that (7*10^k + 143)/3 is prime. 0

%I

%S 1,2,3,6,7,10,11,25,26,32,122,123,126,161,292,320,743,1630,2738,3178,

%T 4814,4833,5030,7035,8151,12554,13954,15113,80490,96112,121487,190683

%N Numbers k such that (7*10^k + 143)/3 is prime.

%C For n>1, numbers such that the digit 2 followed by n-2 occurrences of the digit 3 followed by the digits 81 is prime (see Example section).

%C a(33) > 2*10^5.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/primedifficulty.txt">Search for 23w81.</a>

%e 3 is in this sequence because (7*10^3 + 143)/3 = 2381 is prime.

%e Initial terms and primes associated:

%e a(1) = 1, 71;

%e a(2) = 2, 281;

%e a(3) = 3, 2381;

%e a(4) = 6, 2333381;

%e a(5) = 7, 23333381, etc.

%t Select[Range[0, 100000], PrimeQ[(7*10^# + 143)/3] &]

%o (PARI) is(n)=ispseudoprime((7*10^n+143)/3) \\ _Charles R Greathouse IV_, Jun 13 2017

%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.

%K nonn,more

%O 1,2

%A _Robert Price_, Apr 10 2016

%E a(31)-a(32) from _Robert Price_, Mar 30 2018

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Last modified January 20 22:57 EST 2020. Contains 331104 sequences. (Running on oeis4.)