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Powers in a Commutative Ring, a Field and an Ordered Field: Exponent Laws, Factorisation, Geometric Sums, Monotonicity and Bernoulli's Inequality

Powers satisfy the exponent laws and the factorisation of xnx^n - yny^n; in a field the powers of a nonzero element are nonzero and the geometric sum formula holds; in an ordered field powers of nonnegative elements are nonnegative and strictly increasing in the base, monotone in the exponent for bases at least 1 or between 0 and 1, and Bernoulli's inequality holds.

Statement

In the setting of Commutative Rings, Fields and Ordered Fields: Standard Notation, let RR, with ++, ⋅\cdot, 00 and 11, be a commutative ring, and let x,y∈Rx,y\in R and m,n∈N0m,n\in\mathbb{N}_{0}.

xm+n=xmxnx^{m+n}=x^{m}x^{n} and (xm)n=xmn(x^{m})^{n}=x^{mn}.

(xy)n=xnyn(xy)^{n}=x^{n}y^{n}, x1=xx^{1}=x and 1n=11^{n}=1; if n≥1n\ge1, then 0n=00^{n}=0.

If n≥1n\ge1, then xn−yn=(x−y)∑k=1nxk−1yn−kx^{n}-y^{n}=(x-y)\sum_{k=1}^{n}x^{k-1}y^{n-k}.

Let now FF be a field; let x∈Fx\in F and n∈N0n\in\mathbb{N}_{0}.

If x≠0x\neq0, then xn≠0x^{n}\neq0 and (x−1)n=(xn)−1(x^{-1})^{n}=(x^{n})^{-1}.

If x≠1x\neq1 and n≥1n\ge1, then ∑k=1nxk−1=1−xn1−x\sum_{k=1}^{n}x^{k-1}=\dfrac{1-x^{n}}{1-x}.

Let now FF be an ordered field; let x,y∈Fx,y\in F and n∈N0n\in\mathbb{N}_{0}.

If x≥0x\ge0, then xn≥0x^{n}\ge0; if x>0x>0, then xn>0x^{n}>0; and ∣xn∣=∣x∣n|x^{n}|=|x|^{n}.

If n≥1n\ge1 and 0≤x<y0\le x<y, then xn<ynx^{n}<y^{n}.

If n≥1n\ge1 and x,y≥0x,y\ge0, then x≤yx\le y if and only if xn≤ynx^{n}\le y^{n}, and x=yx=y if and only if xn=ynx^{n}=y^{n}.

If 0≤x≤10\le x\le1, then 0≤xn+1≤xn≤10\le x^{n+1}\le x^{n}\le1.

If x≥1x\ge1, then 1≤xn≤xn+11\le x^{n}\le x^{n+1}.

Let m∈N0m\in\mathbb{N}_{0} with m≤nm\le n. If x≥1x\ge1, then xm≤xnx^{m}\le x^{n}; if 0≤x≤10\le x\le1, then xn≤xmx^{n}\le x^{m}; and if x>1x>1 and m<nm<n, then xm<xnx^{m}<x^{n}.

If x≥−1x\ge-1, then (1+x)n≥1+n x(1+x)^{n}\ge1+n\,x.

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