A-M baby

Task/Gamma-function/Common-Lisp/gamma-function.lisp

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+; Taylor series coefficients
+(defconstant tcoeff
+  '( 1.00000000000000000000  0.57721566490153286061 -0.65587807152025388108
+    -0.04200263503409523553  0.16653861138229148950 -0.04219773455554433675
+    -0.00962197152787697356  0.00721894324666309954 -0.00116516759185906511
+    -0.00021524167411495097  0.00012805028238811619 -0.00002013485478078824
+    -0.00000125049348214267  0.00000113302723198170 -0.00000020563384169776
+     0.00000000611609510448  0.00000000500200764447 -0.00000000118127457049
+     0.00000000010434267117  0.00000000000778226344 -0.00000000000369680562
+     0.00000000000051003703 -0.00000000000002058326 -0.00000000000000534812
+     0.00000000000000122678 -0.00000000000000011813  0.00000000000000000119
+     0.00000000000000000141 -0.00000000000000000023  0.00000000000000000002))
+
+; number of coefficients
+(defconstant numcoeff (length tcoeff))
+
+(defun gamma (x)
+  (let ((y (- x 1.0))
+        (sum (nth (- numcoeff 1) tcoeff)))
+    (loop for i from (- numcoeff 2) downto 0 do
+          (setf sum (+ (* sum y) (nth i tcoeff))))
+    (/ 1.0 sum)))
+
+(loop for i from 1 to 10
+   do (
+     format t "~12,10f~%" (gamma (/ i 3.0))))