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A290341
Product of k in [1, 2n-1] with k and k+1 coprime to 2n-1.
1
1, 1, 6, 120, 28, 362880, 39916800, 91, 1307674368000, 355687428096000, 12160, 51090942171709440000, 179977838616576, 608608000, 10888869450418352160768000000, 8841761993739701954543616000000, 2401235200, 13359987584925696, 10333147966386144929666651337523200000000, 2044818012160
OFFSET
1,3
COMMENTS
If 2n-1 is prime, a(n) = (2n-3)!.
a(n) == 1 (mod 2n-1).
EXAMPLE
For n=5, the allowed k are 1, 4, 7, so a(5) = 1*4*7=28.
MAPLE
seq(convert(select(t -> igcd(t*(t+1), m)=1, [$1..m-1]), `*`), m = 1..100, 2);
CROSSREFS
Sequence in context: A373239 A271648 A353691 * A246827 A127726 A117063
KEYWORD
nonn
AUTHOR
Robert Israel, Jul 27 2017
STATUS
approved