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12, 490, 17136, 584988, 19889100, 675741430, 22955884992, 779827644120, 26491203224556, 899921193951778, 30570830043692400, 1038508304094967860, 35278711531352926572, 1198437683891107427950, 40711602541519349266176, 1382996048732155862584368
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OFFSET
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1,1
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COMMENTS
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Solution to a*b = (b*(b-1) - a*(a+1))/2 in A000027 with a,b >= 2.
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LINKS
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FORMULA
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G.f.: 2*x*(2 - x)*(3 + x) / ((1 - x)*(1 - 34*x + x^2)*(1 - 6*x + x^2)).
a(n) = 41*a(n-1) - 246*a(n-2) + 246*a(n-3) - 41*a(n-4) + a(n-5) for n>5.
(End)
a(n) = (1 - 8*U(n, 3) + 7*U(n, 17) + U(n-1, 17)) / 16 where U(n, x) is the Chebyshev polynomial of the second kind. - Michael Somos, Jul 18 2018
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EXAMPLE
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A053141(2) = 14 and A001109(3) = 35, then 14*35 = 15+16+17+18+19+20+21+22+23+24+25+26+27+28+29+30+31+32+33+34 = 490, is a term.
G.f. = 12*x + 490*x^2 + 17136*x^3 + 584988*x^4 + 19889100*x^5 + ... - Michael Somos, Jul 18 2018
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MATHEMATICA
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lst={}; k=1; j=0; s=1; Do[a=6*k-j; p=2*s*a; s=s+a; AppendTo[lst, p]; j=k; k=a, {n, 1, 16}]; lst
LinearRecurrence[{41, -246, 246, -41, 1}, {12, 490, 17136, 584988, 19889100}, 30] (* G. C. Greubel, Jul 15 2018 *)
a[ n_] := (1 - 8 ChebyshevU[n, 3] + 7 ChebyshevU[n, 17] + ChebyshevU[n - 1, 17]) / 16; (* Michael Somos, Jul 18 2018 *)
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PROG
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(PARI) Vec(2*x*(2 - x)*(3 + x) / ((1 - x)*(1 - 34*x + x^2)*(1 - 6*x + x^2)) + O(x^20)) \\ Colin Barker, Mar 31 2018
(PARI) {a(n) = if( n>0, polcoeff( 2*x * (2 - x)*(3 + x) / ((1 - x)*(1 - 34*x + x^2)*(1 - 6*x + x^2)) + x * O(x^n), n), n=-n; polcoeff( -2*x^2 * (1-2*x)*(1+3*x) / ((1 - x)*(1 - 34*x + x^2)*(1 - 6*x + x^2)) + x * O(x^n), n))}; /* Michael Somos, Jul 18 2018 */
(Magma) m:=25; R<x>:=PowerSeriesRing(Integers(), m); Coefficients(R!(2*x*(2-x)*(3+x)/((1-x)*(1-34*x+x^2)*(1-6*x+x^2)))); // G. C. Greubel, Jul 15 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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EXTENSIONS
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STATUS
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approved
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