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Finite-site microcanonical and canonical block equivalence
Statement
Let , , have real energies , and fix finite . Set , , and . For each , let and . Define the shell and the uniform law on that shell. For probability laws on the finite set , total variation distance means . Then is nonempty for every ; for each fixed , the first -site marginal (for ) converges in that distance to ; and The conclusion is for this finite alphabet, additive energy, exact shell width and uniform shell law. It does not assert finite-size equality or equivalence for interacting or continuous models.
Facts & Assumptions
Given: The finite-site canonical law and uniform shell convention.
is finite, , , , and with .
Finite-site canonical law and microcanonical shell: configurations have additive energies, the canonical probabilities are , and the shell law is uniform when the shell is nonempty.
Finite sums and finite products, by recursion and Order, continuity, range, and the product, quotient, and reciprocal laws for the natural logarithm: the finite sums and logarithms obey the usual finite distributive and product identities.
The real exponential function and the number by a power series and The natural logarithm as the inverse of the exponential function: exponential weights are positive and logarithm is the inverse of the positive real exponential.
The natural logarithm has derivative 1/x and equals the integral from 1 to x of 1/t: is continuous on , has derivative , and .
Heine-Borel in : with the Euclidean metric a subset of is compact if and only if it is closed and bounded, and the proof by bisection uses no choice principle; the same holds on the real line: the finite-dimensional probability simplex is closed and bounded, hence compact; closed bounded subsets of finite-dimensional Euclidean space are compact.
A continuous real-valued function on a nonempty compact metric space is bounded and attains a greatest and a least value: a continuous real-valued function on a nonempty compact metric space attains its minimum and maximum.
Proof
Given: [A1] and the conventions in [F1].
For a type , the number of configurations of that type is . The increasing function gives Evaluating the antiderivative and using shows with an absolute constant; the case uses and . Subtracting the denominator estimates from the numerator estimate gives a constant such that where . The sharper upper bound follows by expanding and retaining the nonnegative terms with counts . For later limits, [F4] gives for , hence .
For any probability vector , define . From [F4], for , with equality only at : apply to and negate. Therefore Since every , equality forces every index into the support and for each , so . Also , hence . In particular, among types of exact mean energy , entropy is at most , with equality only at .
Round down for and put the remainder in . Then for and . Thus This type belongs to , proving nonemptiness. Its frequencies tend to . For , [F4] gives by integrating on ; hence extends continuously by at and . By the lower bound in 1.1, the shell contains at least configurations.
Fix and let and . The simplex is closed and bounded, and is closed in it, so both are compact by [F5]. If , the claim is vacuous. Otherwise let , a closed subset of . If , the continuous function has a positive minimum on by [F6], so choose smaller than this minimum and any . If , [F6] gives a maximum of on ; step 1.2 gives . Put and let , a relative open neighborhood of . The compact set , if nonempty, has a positive minimum of by [F6]; choose . If , choose any . In either case, every with satisfies . For large , ; therefore every shell type outside the -ball has entropy at most . There are at most types, and 1.1 bounds the number of configurations of each by . By the log-growth estimate in 1.1, this type-count factor is subexponential, so division by the shell lower bound in step 2.1 shows that the shell probability outside the ball tends to zero exponentially. Thus the random type converges in probability to .
Conditional on a type , a uniform configuration is a uniformly ordered list of its labeled positions. Couple the first draws without replacement with independent uniform position draws: if the independent positions are distinct, keep them; if not, replace them by an independently chosen uniform ordered distinct -tuple. The resulting tuple is uniform among distinct positions and agrees with the original draws except on the collision event. A union bound over the pairs gives collision probability at most . Hence the type-conditional label marginal differs in total variation from by at most this quantity. Since in probability, for each fixed tuple the random variable tends to zero in probability and is bounded by . For every , , so ; summing over the finite set proves the marginal converges to in total variation.
The rounded shell type and lower bound in step 2.1 give . For any shell type , step 1.2 gives the exact identity There are at most types, and step 1.1 bounds each multiplicity by . Therefore By the log-growth estimate in 1.1, . Since also , the limsup is at most . Together with the lower bound this proves the entropy-density limit, including , without invoking a nonexistent compactness step.
Depends on
Finite sums and finite products, by recursion
The natural logarithm as the inverse of the exponential function
The real exponential function and the number by a power series
Finite-site canonical law and microcanonical shell
A continuous real-valued function on a nonempty compact metric space is bounded and attains a greatest and a least value
Heine-Borel in : with the Euclidean metric a subset of is compact if and only if it is closed and bounded, and the proof by bisection uses no choice principle; the same holds on the real line
The natural logarithm has derivative 1/x and equals the integral from 1 to x of 1/t
Order, continuity, range, and the product, quotient, and reciprocal laws for the natural logarithm