Is $\sum a$ a customary (standard) shorthand for $\sum_{i\in\operatorname{dom}a} a_i$, where $a$ is an indexed family of say integers?
Sum without an index
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$\begingroup$
notation
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5Duplicate of [124354](http://math.stackexchange.com/questions/124354/what-does-sum-mean-without-a-starting-index-and-limit/124355#124355)? – 2012-03-27
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0I have never seen such a notation. Only $\sum_1^n a=na$ (with $a$ constant) or e. g. $\sum_{i\in\operatorname{dom}A} a_i=a_1+a_2+\dots a_n$, with $A=\{a_1,a_2,\dots,a_n\}$. But you mean something different, don't you? – 2012-03-27
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1http://math.stackexchange.com/questions/124322/a-contradiction-in-notation#comment287236_124322 – 2012-03-27