1 files changed, 9 insertions, 0 deletions
diff --git a/src/approx/derivative.lisp b/src/approx/derivative.lisp
new file mode 100644
index 0000000..02fcb4c
--- /dev/null
+++ b/src/approx/derivative.lisp
@@ -0,0 +1,9 @@
+(in-package :lizfcm.approx)
+
+(defun derivative-at (f x &optional (delta 0.01))
+  (let* ((x2 (+ x delta))
+         (x1 (- x delta))
+         (y2 (apply f (list x2)))
+         (y1 (apply f (list x1))))
+    (/ (- y2 y1)
+       (- x2 x1))))