# Summation notation with an increment

I want to place a number or character inside $\sum$ to indicate the increment. So for example $\sum_{i=1}^8 i$ with 3 inside $\sum$ will indicate 1+4+7. How would I accomplish this?

This is to follow a convention in most programming languages that allow an array of equally spaced numbers to be created by using a syntax similar to start:increment:end.

Of course the index of summation can be redefined so as to accommodate any desired fix number but the readability may suffer.

• I am not sure why you do not write $\sum_{i=0}^3 (2i+1)$. Or is that what you want to exclude with your last paragraph in the question? – Lucas Jul 22 '18 at 22:20
• @Lucas Yes, while redefining the running index would be perfectly OK sometimes one ends up with expressions such as $\floor*{\frac{N}{m}}$ as the top value of summation and other artifacts. – Maesumi Jul 24 '18 at 15:53

\documentclass[11pt]{article}
$\sum_{\substack{i = 1\\\text{step }3}}^8 i$%