Plural form of index: INDEXES OR INDICES?
Quoted from https://business.nasdaq.com/marketinsite/2016/Indexes-or-Indices-Whats-the-deal.html:
Both "indexes" and "indices" are acceptable plural forms of the word "index" or to refer to more than one index. Index is one of those rare words that have two different plurals in English. "Indices" is originally a Latin plural, while "Indexes" has taken the English way of making plurals, using –s or –es. Though both are still widely used, they take on different usage in their senses. "Indices" is used when referring to mathematical, scientific and statistical contexts.
The reason for posting this is because I have a question:
What if we switch the indices of summation like \( \sum\limits_{j=1}^n\sum\limits_{i=1}^j (3i+7j)\)?