By Giuliana Davidoff, Peter Sarnak, Alain Valette

This article is a self-contained learn of expander graphs, in particular, their specific development. Expander graphs are hugely hooked up yet sparse, and whereas being of curiosity inside combinatorics and graph concept, they could even be utilized to machine technological know-how and engineering. just a wisdom of hassle-free algebra, research and combinatorics is needed as the authors give you the useful history from graph thought, quantity thought, staff idea and illustration thought. therefore the textual content can be utilized as a quick creation to those topics and their synthesis in smooth arithmetic.

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**Additional resources for Elementary number theory, group theory, and Ramanujan graphs**

**Sample text**

Un x2 is equal to the space of polynomials of degree at most n. Hence we have that L lim m→∞ −L p(x) dνm (x) = L p(x) dν(x) −L for any polynomial p(x). The rest of the argument is a standard 3ε reasoning: fix a continuous function f on [−L , L], and a positive number ε > 0. By the Weierstrass approximation theorem, find a polynomial p such that | f (x) − p(x)| ≤ ε for every x ∈ [−L , L]. Then L ≤ + −L L −L L −L f (x) dνm (x) − L f (x) dν(x) −L ( f (x) − p(x)) dνm (x) + ( p(x) − f (x)) dν(x) .

Finally, X m−1 (αm ) αm = −X m (αm ) = 0, by definition of αm . Third Step. Set Ym (x) = X m (x)2 ; x−αm 2m−1 then Ym = m−2 all i=1 X m−2 (αm ) − yi X i , with yi ≥ 0. i=0 Indeed, by the second step we have Ym = m−1 X m−1−i (αm ) X i X m . Now i=0 π increases to 2. So for j < m : observe that the sequence αm = 2 cos m+1 X j (αm ) > 0 (since αm > α j and α j is the largest root of X j ). This means that all coefficients are positive in the previous formula for Ym . By the first step, each X i X m is a linear combination, with nonnegative coefficients, of X 0 , X 1 , .

4). (a) N (π ) p (⇐) Observe that, if N (π) is prime in Z, then π is prime in Z [i]. Indeed, if π = αβ, taking norms we get N (π) = N (α) N (β), which gives immediately that either α or β is invertible. So, if either N (π) = 2 or N (π) = p, with p ≡ 1 (mod. 4), then π is prime in Z [i]. On the other hand, if q is prime in Z, q ≡ 3 (mod. 7, we cannot have N (α) = N (β) = q. Therefore, either α or β is a unit in Z [i]. With this in hand, we now reach Legendre’s formula for r2 (n), for which we will need some additional notation.