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A295395
Numbers k such that (61*10^k - 1)/3 is prime.
0
3, 17, 18, 30, 78, 98, 371, 947, 1085, 1560, 1607, 2145, 4310, 4637, 16674, 17148, 25209, 27929, 33491, 74189, 124809, 142446
OFFSET
1,1
COMMENTS
Numbers k such that the digits 20 followed by k occurrences of the digit 3 is prime (see Example section).
a(23) > 2*10^5.
EXAMPLE
3 is in this sequence because (61*10^3 - 1)/3 = 20333 is prime.
Initial terms and associated primes:
a(1) = 3, 20333;
a(2) = 17, 2033333333333333333;
a(3) = 18, 20333333333333333333;
a(4) = 30, 20333333333333333333333333333333; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(61*10^# - 1)/3] &]
PROG
(PARI) isok(k) = isprime((61*10^k - 1)/3); \\ Michel Marcus, Nov 22 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 21 2017
EXTENSIONS
a(21)-a(22) from Robert Price, Jan 05 2020
STATUS
approved