Paper V visual guide · Visual 34 of 40 · LOCATION STABILITY / A ONE-SIXTEENTH DISPLACEMENT

A shifted step and a shifted ramp have different moduli

Traditional monograph: Corollary E8.2 (p. 60), Theorem E8.3 (p. 60), Theorem E8.7 (p. 62).

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Set \(\tau=1/4\), \(\widehat\tau=5/16\), and \(\delta=1/16\). A unit step differs only on the interval between its two locations:

\[ \|H_\tau-H_{\widehat\tau}\|_2=\sqrt\delta=\tfrac14, \qquad \|H_\tau-H_{\widehat\tau}\|_\infty=1. \]
Displacing a step versus displacing a ramp
Original drawing. Read the description below; expand the labels and data for selectable text.

Reading the drawing

Two panels share the time variable but have different vertical scales. The upper step error has height one; the lower ramp error has height one sixteenth.

Drawing labels and data 12 labels

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  • \(t\)
  • \(1\)
  • step error
  • \(t\)
  • \(1/16\)
  • ramp error
  • \(1/4\)
  • \(5/16\)
  • \(1\)
  • \(1/4\)
  • \(5/16\)
  • \(1\)

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For a ramp, the error height shrinks too

Write \(r_\tau(t)=(t-\tau)_+\). The difference grows linearly from zero to \(\delta\) on \([\tau,\widehat\tau]\) and stays at \(\delta\) on \([\widehat\tau,1]\). Direct integration gives

\[ \|r_\tau-r_{\widehat\tau}\|_2^2 =\frac{\delta^3}{3}+(1-\widehat\tau)\delta^2 =\frac{17}{6144}<\frac1{256}=\delta^2. \]

Thus the \(L^2\) ramp error is at most \(\delta\), and its supremum error is exactly \(\delta\). Positive-order truncated powers are Lipschitz in location, with the order-dependent constants supplied by the theorem.

The geometric analogue needs a tube hypothesis

If \(E\triangle F\subseteq N_\delta(\partial E)\) and the tube measure is bounded by \(C\delta\) for the stated range \(0<\delta<\delta_0\), then an equal-amplitude packet has finite-\(p\) error at most \(|a|(C\delta)^{1/p}\). A mere visual impression that two boundaries are close does not establish the inclusion or the tube estimate.

Read this as

A displaced value jump keeps its full height on a smaller set. A displaced derivative-jump packet also decreases in height.

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