September 2017 Update

Task/Gamma-function/Kotlin/gamma-function.kotlin

@@ -0,0 +1,35 @@
+// version 1.0.6
+
+fun gammaStirling(x: Double): Double = Math.sqrt(2.0 * Math.PI / x) * Math.pow(x / Math.E, x)
+
+fun gammaLanczos(x: Double): Double {
+    var xx = x
+    val p = doubleArrayOf(
+        0.99999999999980993,
+      676.5203681218851,
+    -1259.1392167224028,			     	
+      771.32342877765313,
+     -176.61502916214059,
+       12.507343278686905,
+       -0.13857109526572012,
+        9.9843695780195716e-6,
+        1.5056327351493116e-7
+    )
+    val g = 7
+    if (xx < 0.5) return Math.PI / (Math.sin(Math.PI * xx) * gammaLanczos(1.0 - xx))
+    xx--
+    var a = p[0]
+    val t = xx + g + 0.5
+    for (i in 1 until p.size) a += p[i] / (xx + i)
+    return Math.sqrt(2.0 * Math.PI) * Math.pow(t, xx + 0.5) * Math.exp(-t) * a
+}
+
+fun main(args: Array<String>) {
+    println(" x\tStirling\t\tLanczos\n")
+    for (i in 1 .. 20) {
+        val d = i / 10.0
+        print("%4.2f\t".format(d))
+        print("%17.15f\t".format(gammaStirling(d)))
+        println("%17.15f".format(gammaLanczos(d)))
+    }
+}