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A305862
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a(n) = 384*4^n - 576*3^n + 220*2^n - 14.
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3
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14, 234, 1826, 10770, 55154, 260274, 1167026, 5059890, 21442994, 89438514, 368866226, 1509026610, 6137242034, 24853275954, 100327829426, 404059098930, 1624486948274, 6522713868594, 26165182536626, 104883769004850, 420204307937714, 1682825158192434, 6737324873467826
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OFFSET
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0,1
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COMMENTS
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a(0) = 2*7 and a(40) = 2*232110255958477539427146457 are semiprimes. For which values of n > 40 is a(n) semiprime?
For odd n, a(n) is divisible by 2*3.
For n == 3 (mod 4), a(n) is divisible by 2*3*5.
For n == 0 or 5 (mod 6), a(n) is divisible by 2*7.
For n == 2 or 4 (mod 5), a(n) is divisible by 2*11.
For n == 1 or 11 (mod 12), a(n) is divisible by 2*3*13.
For n == 15 (mod 16), a(n) is divisible by 2*3*5*17^2, etc.
If a(n) is divisible by 37 then it is also divisible by 3*5*7*13*19*73. (End)
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LINKS
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FORMULA
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G.f.: 2*(7 + 47*x - 12*x^2)/((1 - x)*(1 - 2*x)*(1 - 3*x)*(1 - 4*x)).
a(n) = 10*a(n-1) - 35*a(n-2) + 50*a(n-3) - 24*a(n-4).
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MATHEMATICA
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Table[384 4^n - 576 3^n + 220 2^n - 14, {n, 0, 30}]
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PROG
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(Magma) [384*4^n-576*3^n+220*2^n-14: n in [0..30]];
(PARI) a(n) = 384*4^n - 576*3^n + 220*2^n - 14; \\ Michel Marcus, Jul 03 2018
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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