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Weight-one counting misses Heisenberg growth

Statement refuted

The claim that every independent lower-central coordinate in every word ball of a finitely generated nilpotent group has range bounded linearly in the radius is false. Counting all such coordinates with weight one can therefore give the wrong polynomial degree.

Facts & Assumptions

Given: Use H with horizontal generators x,y, central z=[x,y], and unique normal form x^a y^b z^k.

[F1]

H has three independent normal coordinates and growth degree four (The discrete Heisenberg group has growth degree four).

[F2]

[xa,yb]=zab, with central exponent attainable at length O(sqrt of its absolute value) (The Heisenberg center is quadratically distorted).

Counterexample

1.1

For every integer N1 the explicit word [xN,yN]=xNyNxNyN=zN2 has length at most 4N. Its normal coordinate tuple is (0,0,N^2), since the ordered form is unique. Hence the central coordinate in B(4N) can equal N^2.

F1F2
2.1

If a fixed linear bound kCn held in every ball B(n), step 1.1 would imply N24CN for every positive integer N. Taking any integer N>4C contradicts this. Thus the claimed uniform linear bound fails. Moreover H has three independent coordinates but ball growth degree four by F1, so the predicted degree three from weight-one counting is false; positive multiples of n^4 cannot be bounded above by a fixed multiple of n^3 as n grows.

F1step 1.1

Source notes

Druţu–Kapovich, Lectures on Geometric Group Theory (585-page draft), Corollary 12.39 and Theorem 12.48, pp.322–323,328–329. Draft Corollary 12.39 supplies the central compression phenomenon. The actual refutation uses a concrete commutator word and quantifies against every possible linear constant.

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Sources