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A187371 Least odd number k such that (k*2^n-1)*k*2^n-1 or (k*2^n-1)*k*2^n+1 or (k*2^n+1)*k*2^n-1 or (k*2^n+1)*k*2^n+1 is prime. 5
1, 1, 1, 1, 1, 1, 3, 3, 1, 1, 5, 5, 3, 9, 7, 1, 5, 1, 3, 19, 9, 11, 7, 1, 11, 1, 27, 25, 9, 11, 3, 1, 21, 19, 1, 19, 3, 1, 7, 9, 9, 3, 27, 5, 1, 5, 5, 7, 3, 1, 41, 1, 9, 17, 1, 11, 1, 27, 3, 105, 33, 15, 23, 29, 19, 43, 37, 15, 3, 11, 27, 9, 57, 5, 3, 3, 35 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,7
COMMENTS
As N increases, it appears that (sum_{k=1..N} a(k)) / (sum_{k=1..N} k) tends to 0.24
LINKS
FORMULA
a(n) = least of A187367(n), A187368(n), A187369(n), A187370(n).
MATHEMATICA
Table[k = 1; While[! PrimeQ[(k*2^n - 1)*k*2^n - 1] && ! PrimeQ[(k*2^n - 1)*k*2^n + 1] && ! PrimeQ[(k*2^n + 1)*k*2^n - 1] && ! PrimeQ[(k*2^n + 1)*k*2^n + 1], k = k + 2]; k, {n, 100}]
CROSSREFS
Sequence in context: A046218 A046221 A220837 * A056611 A177407 A135669
KEYWORD
nonn
AUTHOR
Pierre CAMI, Mar 09 2011
STATUS
approved

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Last modified March 18 22:50 EDT 2024. Contains 370951 sequences. (Running on oeis4.)