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A158480
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a(n) = 12*n^2 + 1.
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2
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1, 13, 49, 109, 193, 301, 433, 589, 769, 973, 1201, 1453, 1729, 2029, 2353, 2701, 3073, 3469, 3889, 4333, 4801, 5293, 5809, 6349, 6913, 7501, 8113, 8749, 9409, 10093, 10801, 11533, 12289, 13069, 13873, 14701, 15553, 16429, 17329, 18253, 19201, 20173, 21169
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OFFSET
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0,2
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COMMENTS
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The identity (12*n^2 + 1)^2 - (36*n^2 + 6)*(2*n)^2 = 1 can be written as a(n)^2 - A158479(n)*A005843(n)^2 = 1.
Sequence found by reading the line from 13, in the direction 13, 49, ..., in the square spiral whose vertices are the generalized octagonal numbers A001082. - Omar E. Pol, Jul 18 2012
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LINKS
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FORMULA
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G.f: (13*x^2 + 10*x + 1)/(1-x)^3.
a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3).
Sum_{n>=0} 1/a(n) = (1 + (Pi/sqrt(12))*coth(Pi/sqrt(12)))/2.
Sum_{n>=0} (-1)^n/a(n) = (1 + (Pi/sqrt(12))*csch(Pi/sqrt(12)))/2. (End)
Product_{n>=0} (1 + 1/a(n)) = sqrt(2)*csch(Pi/sqrt(12))*sinh(Pi/sqrt(6)).
Product_{n>=1} (1 - 1/a(n)) = (Pi/sqrt(12))*csch(Pi/sqrt(12)). (End)
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EXAMPLE
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a(1) = 12*1^2 + 1 = 13.
a(2) = 12*2^2 + 1 = 49.
a(3) = 12*3^2 + 1 = 109.
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MAPLE
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MATHEMATICA
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LinearRecurrence[{3, -3, 1}, {13, 49, 109}, 40]
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PROG
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(Magma) I:=[13, 49, 109]; [n le 3 select I[n] else 3*Self(n-1)-3*Self(n-2) +Self(n-3): n in [1..50]];
(PARI) a(n)=12*n^2+1
(GAP) List([1..40], n-> 12*n^2 + 1); # G. C. Greubel, Nov 06 2019
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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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EXTENSIONS
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STATUS
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approved
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