32 lines
825 B
JavaScript
32 lines
825 B
JavaScript
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/*
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* A gamma function implementation based on Lanczos Approximation
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* https://en.wikipedia.org/wiki/Lanczos_approximation
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*/
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var Complex = require('../complex');
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var P = [Complex(0.99999999999980993),
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Complex(676.5203681218851), Complex(-1259.1392167224028), Complex(771.32342877765313),
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Complex(-176.61502916214059), Complex(12.507343278686905), Complex(-0.13857109526572012),
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Complex(9.9843695780195716e-6), Complex(1.5056327351493116e-7)];
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var SQRT2PI = Complex(Math.sqrt(2 * Math.PI));
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function gamma(z) {
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z = z.sub(1);
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var x = P[0];
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var t = z.add(7.5);
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for (var i = 1; i < P.length; i++) {
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x = x.add(P[i].div(z.add(i)));
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}
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return SQRT2PI.mul(t.pow(z.add(0.5))).mul(t.neg().exp()).mul(x);
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}
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var fac = 1;
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for (var i = 1; i <= 10; i++) {
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console.log(fac, gamma(Complex(i)));
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fac *= i;
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}
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