# What sums should everyone know?

Well, everyone is an overstatement. Qualifying it: I'm interested in algorithms (complexity) and probabilistic processes/models and I often see sums converge to a certain value in the books I'm reading (algorithm iterations and finite and infinite series of probabilities etc.). I'm looking for a reference (website for example) that contains common sums and their convergence values (infinite or finite) that are often times encountered in these subjects.

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Gradshteyn and Ryzhik's Table of Integrals, Series, and Products may be useful –  M. Strochyk Sep 25 '12 at 8:00
This possibly contains all you need. –  David Mitra Sep 25 '12 at 14:13

Geometric Series and its derivatives are very common in complexity problems as well as probabilistic models.

$$a+ar+ar^2+\cdots+ar^{n-1}=\frac{a(r^n-1)}{r-1}$$

For $|r|<1,$ infinite geometric series converges

$$a+ar+ar^2+\cdots =\frac{a}{1-r}$$

Binomial expansion is also common

$$(a+b)^n={n\choose 0}a^{n}b^{0}+{n\choose 1}a^{n-1}b^{1}+\cdots+{n\choose k}a^{n-k}b^{k}+\cdots +{n\choose n}a^{0}b^{n}$$

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