36 lines
1008 B
Kotlin
36 lines
1008 B
Kotlin
// version 1.0.6
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fun gammaStirling(x: Double): Double = Math.sqrt(2.0 * Math.PI / x) * Math.pow(x / Math.E, x)
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fun gammaLanczos(x: Double): Double {
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var xx = x
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val p = doubleArrayOf(
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0.99999999999980993,
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676.5203681218851,
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-1259.1392167224028,
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771.32342877765313,
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-176.61502916214059,
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12.507343278686905,
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-0.13857109526572012,
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9.9843695780195716e-6,
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1.5056327351493116e-7
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)
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val g = 7
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if (xx < 0.5) return Math.PI / (Math.sin(Math.PI * xx) * gammaLanczos(1.0 - xx))
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xx--
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var a = p[0]
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val t = xx + g + 0.5
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for (i in 1 until p.size) a += p[i] / (xx + i)
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return Math.sqrt(2.0 * Math.PI) * Math.pow(t, xx + 0.5) * Math.exp(-t) * a
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}
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fun main(args: Array<String>) {
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println(" x\tStirling\t\tLanczos\n")
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for (i in 1 .. 20) {
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val d = i / 10.0
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print("%4.2f\t".format(d))
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print("%17.15f\t".format(gammaStirling(d)))
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println("%17.15f".format(gammaLanczos(d)))
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}
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}
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