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A271625
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a(n) = 2n^2 + 4n - 3.
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4
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3, 13, 27, 45, 67, 93, 123, 157, 195, 237, 283, 333, 387, 445, 507, 573, 643, 717, 795, 877, 963, 1053, 1147, 1245, 1347, 1453, 1563, 1677, 1795, 1917, 2043, 2173, 2307, 2445, 2587, 2733, 2883, 3037, 3195, 3357, 3523, 3693, 3867, 4045, 4227, 4413, 4603, 4797, 4995, 5197, 5403, 5613, 5827
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OFFSET
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1,1
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COMMENTS
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Numbers n such that 2n + 10 is a perfect square.
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LINKS
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Table of n, a(n) for n=1..53.
Index entries for linear recurrences with constant coefficients, signature (3,-3,1).
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FORMULA
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G.f.: x*(3 + 4*x - 3*x^2)/(1 - x)^3. - Ilya Gutkovskiy, Apr 11 2016
Sum_{n>=1} 1/a(n) = 13/30 - Pi*cot(sqrt(5/2)*Pi)/(2*sqrt(10)) = 0.5627678459924... . - Vaclav Kotesovec, Apr 11 2016
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EXAMPLE
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a(1) = 2*1^2 + 4*1 - 3 = 3.
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MATHEMATICA
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Table[2 n^2 + 4 n - 3, {n, 53}] (* Michael De Vlieger, Apr 11 2016 *)
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PROG
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(Magma) [ 2*n^2 + 4*n - 3: n in [1..60]];
(Magma) [ n: n in [1..6000] | IsSquare(2*n+10)];
(PARI) x='x+O('x^99); Vec(x*(3+4*x-3*x^2)/(1-x)^3) \\ Altug Alkan, Apr 11 2016
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CROSSREFS
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Cf. A201713.
Numbers h such that 2*h + k is a perfect square: A294774 (k=-9), A255843 (k=-8), A271649 (k=-7), A093328 (k=-6), A097080 (k=-5), A271624 (k=-4), A051890 (k=-3), A058331 (k=-2), A001844 (k=-1), A001105 (k=0), A046092 (k=1), A056222 (k=2), A142463 (k=3), A054000 (k=4), A090288 (k=5), A268581 (k=6), A059993 (k=7), (-1)*A147973 (k=8), A139570 (k=9), this sequence (k=10), A222182 (k=11), A152811 (k=12), A181510 (k=13), A161532 (k=14), no sequence (k=15).
Sequence in context: A335747 A066947 A031011 * A099062 A318368 A196014
Adjacent sequences: A271622 A271623 A271624 * A271626 A271627 A271628
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KEYWORD
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nonn,easy
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AUTHOR
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Juri-Stepan Gerasimov, Apr 11 2016
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STATUS
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approved
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