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A128286
a(n) is the n-th smallest product of 5 odd primes m = p1*p2*p3*p4*p5 such that (d+m/d)/2 are all primes for each d dividing m.
4
884037, 1137565, 2398377, 123156993, 681714273, 2347722213, 7283144845, 7794246057, 8953447917, 10287992785, 13749228493, 38108016453, 38901676405, 70918253385, 71809744693, 120418624965, 148282565865, 150721729873
OFFSET
1,1
COMMENTS
a(6) > 10^9.
EXAMPLE
884037 = 3*7*11*43*89 and (1 + 884037)/2, (3 + 7*11*43*89)/2,
(7 + 3*11*43*89)/2, (11 + 3*7*43*89)/2, (43 + 3*7*11*89)/2, (89 + 3*7*11*43)/2,
(3*7 + 11*43*89)/2, (3*11 + 7*43*89)/2, (3*43 + 7*11*89)/2,(3*89 + 7*11*43)/2,
(7*11 + 3*43*89)/2, (7*43 + 3*7*89)/2, (7*89 + 3*7*43)/2, (11*43 + 3*7*89)/2,
(11*89 + 3*7*43)/2, (43*89 + 3*7*11)/2 are all primes and 884037 is the smallest such integer, so a(1) = 884037.
CROSSREFS
KEYWORD
hard,nonn
AUTHOR
Kok Seng Chua (chuakokseng(AT)hotmail.com), Mar 05 2007
EXTENSIONS
a(6)-a(18) from Donovan Johnson, Oct 12 2008
STATUS
approved