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A102926
Smallest prime factor in product of previous terms +1 or -1.
2
2, 3, 5, 29, 11, 7, 13, 37, 17, 79, 23, 4129, 193, 2593, 101, 19, 39163, 577, 26431, 131, 308798542881428667318174028327605372989, 103, 163, 179, 293, 127, 6287, 683437, 31, 89, 13590243019242466336587034391, 113, 2207, 59, 109, 223, 2351
OFFSET
1,1
COMMENTS
A variant of the Euclid-Mullin construction.
This sequence is listed on the OEIS wiki page "OEIS sequences needing factors" and on the corresponding thread on mersenneforum.org. - M. F. Hasler, Mar 21 2013
LINKS
FORMULA
a(n) = least prime factor of b(n)^2-1, where b(n) = product a(k), 0<k<n, = A102927.
EXAMPLE
a(5)=11 because 2*3*5*29=870, 869=11*79, 871=13*67.
a(31) = 13590243019242466336587034391 because this is the least prime factor of A102927(30)+1. The least prime factor of A102927(30)-1 is 44989026625856465412069667987. Remarkably, both are 29-digit numbers. - David Wasserman, Apr 15 2008
MATHEMATICA
spf[{p_, a_}]:=With[{f=FactorInteger[p^2-1][[1, 1]]}, {p*f, f}]; NestList[ spf, {2, 2}, 36][[All, 2]] (* Harvey P. Dale, May 05 2018 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Marc LeBrun, Jan 19 2005
EXTENSIONS
More terms from Don Reble, Jan 23 2005, corrected Sep 26 2006
Further terms from David Wasserman, Apr 15 2008
STATUS
approved