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Minkowski's inequality for finite sums (rational exponent)
Statement
Let , let and be reals, and let with (Order on the rationals). Then
All exponents appearing are positive rationals, so every power is defined for a nonnegative base by Rational powers of a positive base and its supplementary clause .
The conjugate exponent is rational exactly because is. For the proof runs through Hölder with , and a quotient of rationals with nonzero denominator is a rational (Arithmetic on the rationals). Had been an arbitrary real, would still be a real, but would already be undefined: the whole statement lives inside the rational exponents built on this page, as the closing remark of the page explains.
On the case . It reads and follows by summing the two-term triangle inequality (The triangle inequality) termwise. It is not literally the finite-sum triangle inequality Triangle inequality for finite sums, which compares with for one list; combining the two at gives the familiar .
Facts & Assumptions
Given: A natural , reals and , and a rational . Write , , , and when .
Hölder's inequality (Hölder's inequality for finite sums (rational exponents)): for conjugate rationals , .
Laws of finite sums (Laws of finite sums and finite products, Finite sums and finite products, by recursion): additivity, scaling, monotonicity, and the fact that a sum of nonnegative terms is nonnegative.
Rational power laws (Laws of rational exponents, Rational powers of a positive base, Existence and uniqueness of -th roots: a unique with ): for and rationals : , , , and ; and for every rational .
Absolute values (Basic properties of the absolute value, Absolute value in an ordered field, The triangle inequality): , , and .
Rational arithmetic (Arithmetic on the rationals, The rationals as equivalence classes of pairs of integers, Order on the rationals), carried out in the totally ordered field (The rationals form a totally ordered field, which is what makes the order comparisons below legitimate, and which supplies totality, compatibility with addition and closure of the positives under multiplication but NOT ; that is The multiplicative identity is positive, valid in because is an ordered field): for rational one has and, since , also , so the number is a rational with , , and .
Order arithmetic: Order is preserved by adding a constant and by adding inequalities and Sign rules for products and monotonicity of multiplication state adding inequalities and scaling by a positive element for the STRICT order only, so the nonstrict forms used below (adding two , and scaling a by a nonnegative element) are those statements together with the case of equality, which is settled by trichotomy (Ordered field); and the inverse of a positive element is positive (Inverses of positives are positive, and reciprocation reverses order, claim 1).
Proof
Every quantity is defined and nonnegative: , and are nonnegative because , hence so are , and .
The case : summing the two-term triangle inequality termwise and using monotonicity and additivity gives , and since this is exactly the assertion at .
The case : the left-hand side is , which is at most the nonnegative right-hand side.
Assume from now on and , and put , a rational with conjugate to , so that and .
Splitting each term: for one has , valid for by the addition law and for because both sides are ; applying this with and then the triangle inequality, multiplied by the nonnegative factor , gives for every .
The auxiliary list has -th power sum : for by the iterated-power law, and both sides are when ; hence .
Summing the termwise bound: .
Applying Hölder to the pairs and to , and using since : and .
Combining, .
Dividing by , which is legitimate because , and computing , we obtain ; together with the case and the case this proves the inequality for every rational .
Depends on
Hölder's inequality for finite sums (rational exponents)
Finite sums and finite products, by recursion
Laws of finite sums and finite products
Rational powers of a positive base
Laws of rational exponents
Triangle inequality for finite sums
The triangle inequality
Basic properties of the absolute value
Absolute value in an ordered field
Arithmetic on the rationals
Order on the rationals
The rationals as equivalence classes of pairs of integers
The rationals form a totally ordered field
The multiplicative identity is positive
Existence and uniqueness of -th roots: a unique with
Ordered field
Sign rules for products and monotonicity of multiplication
Inverses of positives are positive, and reciprocation reverses order
Order is preserved by adding a constant and by adding inequalities