added 1 definition

Author: JoramSoch

D/const.md

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+---
+layout: definition
+mathjax: true
+
+author: "Joram Soch"
+affiliation: "BCCN Berlin"
+e_mail: "joram.soch@bccn-berlin.de"
+date: 2020-09-09 01:30:00
+
+title: "Constant"
+chapter: "General Theorems"
+section: "Probability theory"
+topic: "Random variables"
+definition: "Constant"
+
+sources:
+  - authors: "ProofWiki"
+    year: 2020
+    title: "Definition: Constant"
+    in: "ProofWiki"
+    pages: "retrieved on 2020-09-09"
+    url: "https://proofwiki.org/wiki/Definition:Constant#Definition"
+
+def_id: "D96"
+shortcut: "const"
+username: "JoramSoch"
+---
+
+
+**Definition:** A constant is a quantity which does not change and thus always has the same value. From a statistical perspective, a constant is a [random variable](/D/rvar) which is equal to its [expected value](/D/mean)
+
+$$ \label{eq:EX}
+X = \mathrm{E}(X)
+$$
+
+or equivalently, whose [variance](/D/var) is zero
+
+$$ \label{eq:VarX}
+\mathrm{Var}(X) = 0 \; .
+$$