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A338477
Numbers k such that 398*k^2 - 1 is prime.
2
1, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 17, 18, 19, 20, 22, 24, 25, 27, 28, 29, 33, 34, 37, 38, 43, 44, 46, 47, 51, 52, 54, 55, 58, 59, 60, 67, 68, 71, 73, 75, 79, 80, 81, 82, 83, 85, 86, 87, 89, 90, 93, 94, 95, 96, 97, 100, 103, 106, 107, 108, 110, 112, 114, 116, 117, 119, 121, 124, 125, 128
OFFSET
1,2
COMMENTS
There are 414 such primes for 1 <= x <= 1000, and 3280 for 1 <= x <= 10000.
FORMULA
a(n) = sqrt(A338476(n) + 1)/398.
EXAMPLE
a(3)=4 is in the sequence because 398*4^2 - 1 = 6367 is prime.
MAPLE
select(t -> isprime(398*t^2-1), [$1..1000]);
CROSSREFS
Cf. A338476.
Cf. A331947, where 398 is a term.
Sequence in context: A026417 A026421 A026481 * A039211 A114035 A329779
KEYWORD
nonn
AUTHOR
Robert Israel, Oct 29 2020
STATUS
approved