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A290115
Numbers k such that (7*10^k + 197)/3 is prime.
0
1, 3, 4, 6, 7, 52, 53, 103, 131, 199, 294, 426, 780, 1144, 1876, 2001, 3507, 5737, 6657, 12558, 28303, 31608, 60643, 74741, 124648
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 3 followed by the digits 99 is prime (see Example section).
a(26) > 2*10^5.
EXAMPLE
4 is in this sequence because (7*10^4 + 197)/3 = 23399 is prime.
Initial terms and associated primes:
a(1) = 1, 89;
a(2) = 3, 2399;
a(3) = 4, 23399;
a(4) = 6, 2333399;
a(5) = 7, 23333399; etc.
MAPLE
select(k -> isprime((7*10^k+197)/3), [$1..10000]); # Robert Israel, Jul 20 2017
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(7*10^# + 197)/3] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jul 19 2017
EXTENSIONS
a(25) from Robert Price, Jul 17 2018
STATUS
approved