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A281642 Numbers k such that 4*10^k - 57 is prime. 0
3, 7, 10, 15, 28, 35, 70, 183, 243, 298, 366, 388, 607, 791, 827, 1704, 1732, 1743, 2755, 2766, 2983, 3312, 7173, 16365, 18234, 102163, 104461, 136813, 154078 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
For k>1, numbers such that the digit 3 followed by k-2 occurrences of the digit 9 followed by the digits 43 is prime (see Example section).
a(30) > 2*10^5.
LINKS
Makoto Kamada, Search for 39w43.
EXAMPLE
3 is in this sequence because 4*10^3 - 57 = 3943 is prime.
Initial terms and primes associated:
a(1) = 3, 3943;
a(2) = 7, 39999943;
a(3) = 10, 39999999943;
a(4) = 15, 3999999999999943;
a(5) = 28, 39999999999999999999999999943; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[4*10^# - 57] &]
CROSSREFS
Sequence in context: A143101 A307612 A330160 * A353427 A121016 A246701
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 25 2017
EXTENSIONS
a(26)-a(29) from Robert Price, Oct 13 2018
STATUS
approved

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Last modified March 29 10:44 EDT 2024. Contains 371268 sequences. (Running on oeis4.)