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A217377 a(n) is the smallest m>=0 such that ((5n+1)*6^m-1)/5 is prime; or -1 if no such value exists. 1
1, 0, 0, 2, 0, 1, 0, 4, 2, 1, 0, 1, 0, 3, 2, 1, 0, 1, 0, 2, 1, 4, 0, 3, 1, 1, 1, 3, 0, 1, 0, 1, 1, 2, 1, 2, 0, 1, 3, 1, 0, 15, 0, 3, 1, 1, 0, 4, 3, 3008, 1, 1, 0, 2, 1, 1, 4, 1, 0, 3, 0, 1, 1, 2, 2, 1, 0, 1, 3, 1, 0, 1, 0, 2, 2, 1, 1, 4, 0, 2, 1, 4, 0, 5, 2, 8 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

COMMENTS

Let f(n)=6n+1. Let f(n,m) be f applied to n m-times. For example f(n,3) = f(f(f(n))). Then a(n) is the smallest m>=0 such that f(n,m) is prime.

a(525)=27871 is the largest found value in this sequence, which generates a probable prime with 21691 digits.

a(1247) and a(1898) are currently unknown. If they are positive then a(1247)>86500 and a(1898)>58000.

LINKS

Dmitri Kamenetsky, Table of n, a(n) for n = 1..1246

EXAMPLE

a(8)=4, because 4 is the smallest value for m such that ((5*8+1)*6^m-1)/5 is prime. The prime value is (41*6^4-1)/5 = 6*(6*(6*(6*8+1)+1)+1)+1 = 10627.

CROSSREFS

Cf. A040081.

Sequence in context: A067631 A134317 A123641 * A276193 A238618 A132277

Adjacent sequences:  A217374 A217375 A217376 * A217378 A217379 A217380

KEYWORD

nonn

AUTHOR

Dmitri Kamenetsky, Oct 01 2012

STATUS

approved

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Last modified October 19 09:31 EDT 2021. Contains 348074 sequences. (Running on oeis4.)