Express dissolved multi-summand binominal equation After dissolving an multi summand binomial equation $$f(x)=(x_0+x_1+x_2+x_3+\ ...)^2$$ I got $n*(n-1)/2$ of these terms: $$x_0x_1+x_0x_2+x_0x_3+x_1x_2+x_1x_3+x_2x_3+...--prophetes.ai

Express dissolved multi-summand binominal equation After dissolving an multi summand binomial equation $$f(x)=(x_0+x_1+x_2+x_3+\ ...)^2$$ I got $n(n-1)/2$ of these terms: $$x_0x_1+x_0x_2+x_0x_3+x_1x_2+x_1x_3+x_2x_3+\ ...$$ How do I write them in a summation formula $\sum$ or other equivalent expressions? Is $$\sum_{i=0}^n x_ix_{i+1}$$ the correct expression?

I suppose that what you are looking for is $$f=\sum _{i=1}^n x_i^2+2 \sum _{i=1}^n x_i \sum _{j=i+1}^n x_j$$