Update README.md

2016-12-15 19:29:11 +01:00 · 2016-12-15 19:29:11 +01:00 · bc851a5995
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@ -4,6 +4,7 @@ An arithmetic derivative calculator that uses these simple formulas:
 * 0' = 1' = 0;
 * if n is a prime, n' = 1;
 * if n = ab  (a and b > 1), n' = a'b + ab'
+* (-n)' = -(n')

 More about arithmetic derivative:
 https://en.wikipedia.org/wiki/Arithmetic_derivative