--- a/probability-discrete.rst	Wed Mar 16 14:00:04 2016 +0200
+++ b/probability-discrete.rst	Wed Mar 16 16:18:11 2016 +0200
@@ -21,6 +21,12 @@
 
    PMF(X = x) = P(X = x) = p_X(x) = P({ω ∈ Ω: X(ω) = x})
 
+   PMF(a ≤ X ≤ b) = P(a ≤ X ≤ b) = ∑_{a ≤ x ≤ b} P(X = x)
+
+   p_X(x) ≥ 0
+
+   ∑_x p_X(x) = 1
+
 where :math:`X` is a random variable on space :math:`Ω` of outcomes which mapped
 to real number via :math:`X(ω)`.