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URL: https://oeis.org/A202449

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A202449
Least k such that k*n!! - 1 is a prime number.
1
3, 2, 1, 1, 2, 1, 4, 1, 2, 3, 2, 3, 2, 6, 16, 1, 6, 7, 6, 6, 6, 3, 26, 3, 2, 1, 16, 3, 8, 8, 4, 15, 16, 3, 2, 6, 4, 6, 14, 9, 38, 19, 6, 8, 28, 2, 32, 43, 44, 4, 24, 8, 28, 35, 20, 2, 62, 11, 4, 26, 10, 10, 16, 1, 72, 13, 10, 2, 2, 4, 2, 23, 18, 3, 32, 12, 132
OFFSET
1,1
COMMENTS
Using Xylouris' version of Linnik's theorem, a(n) << (n/e)^(2.6n). - Charles R Greathouse IV, Dec 22 2011
FORMULA
min{k: k*A006882(n) in A000040}.
EXAMPLE
a(7) = 4 because 4*7!! - 1 = 4*105 - 1 = 419 is prime.
MAPLE
A202449 := proc(n)
for k from 1 do
if isprime(k*doublefactorial(n)-1) then
return k;
end if;
end do:
end proc:
seq(A202449(n), n=1..80) ; # R. J. Mathar, Dec 22 2011
MATHEMATICA
Table[k = 0; While[!PrimeQ[k*n!! -1], k++]; k, {n, 85}]
PROG
(PARI) a(n)=my(N=prod(k=1, n\2, 2*k+n%2), k); while(!isprime(k++*N-1), ); k \\ Charles R Greathouse IV, Dec 22 2011
CROSSREFS
Cf. A202448.
Sequence in context: A083663 A085427 A172130 * A326613 A288627 A232096
KEYWORD
nonn
AUTHOR
Michel Lagneau, Dec 19 2011
EXTENSIONS
a(53), a(54) corrected by Georg Fischer, Mar 25 2025
STATUS
approved