# Ladders, Envelopes, and p-Norms An old technique gives a new approach to an old problem Dan Kalman American University Fall 2014 www.dankalman.net.

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Ladders, Envelopes, and p-Norms An old technique gives a new approach to an old problem Dan Kalman American University Fall 2014 www.dankalman.net

The Ladder Problem: How long a ladder can you carry around a corner?

The Traditional Approach Reverse the question Instead of the longest ladder that will go around the corner … Find the shortest ladder that will not

A Direct Approach Why is this reversal necessary? Look for a direct approach: find the longest ladder that fits Conservative approach: slide the ladder along the walls as far as possible Let’s look at some animations: my machine internet sitemy machineinternet site

About the Boundary Curve Called the envelope of the family of lines Nice calculus technique to find its equation Technique used to be standard topic Well known curve (astroid, hypocycloid) Gives an immediate solution to the ladder problem

Solution to Ladder Problem Ladder will fit if (a,b) is outside the region  Ladder will not fit if (a,b) is inside the region Longest L occurs when (a,b) is on the curve:

A famous curve Hypocycloid: point on a circle rolling within a larger circle Astroid: larger radius four times larger than smaller radius Animated graphic from Mathworld.com

Trammel of Archimedes

Alternate View Ellipse Model: slide a line with its ends on the axes, let a fixed point on the line trace a curve The length of the line is the sum of the semi major and minor axes Animation on next slide

Cool Java Applet

x = a cos  y = b sin 

Family of Ellipses  Paint an ellipse with every point of the ladder  Family of ellipses with sum of major and minor axes equal to length L of ladder  These ellipses sweep out the same region as the moving line  Same envelope

Animated graphic from Mathworld.com

Finding the Envelope Family of curves given by F(x,y,  ) = constant For each  the equation defines a curve Take the partial derivative with respect to  Use the equations of F and F  to eliminate the parameter  Resulting equation in x and y is the envelope

Parameterize Lines L is the length of ladder Parameter is angle  Note x and y intercepts

Find Envelope

Double Parameterization Parameterize line for each  : x(t) = L cos(  )(1-t) y(t) = L sin(  ) t This defines mapping R 2 → R 2 F( ,t) = (L cos(  )(1-t), L sin(  ) t) Fixed   line in family of lines Fixed t  ellipse in family of ellipses Envelope points are on boundary of image: Jacobian F = 0

Mapping R 2 → R 2 Jacobian F vanishes when t = sin 2  Envelope curve parameterized by ( x, y ) = F ( , sin 2  ) = ( L cos 3  L sin 3  )

Another sample family of curves and its envelope

Find parametric equations for the envelope:

Plot those parametric equations:

Doug Ensley’s Envelope Applet

Definition of Envelope Curve tangent to each member of a family of curves Under suitable conditions the boundary of the region swept out meets this definition Observation: Any smooth curve is the envelope for its own family of tangent lines This leads to a nice generalization of the ladder problem. (Joint work with Alan Krinik and Chaitanya Rao.)

Alternate Derivation of x 2/3 + y 2/3 = L 2/3 Begin with the curve x 2/3 + y 2/3 = L 2/3 Consider its family of tangent lines Show that each tangent line intersects the first quadrant in a segment of length L Conclusion: the family of tangent lines is the same as the family of positions of the moving ladder of length L This shows that the envelope of the family of lines has equation x 2/3 + y 2/3 = L 2/3

Details The equation x 2/3 + y 2/3 = L 2/3 defines a level curve of the function f (x,y) = x 2/3 + y 2/3 At any point (s,t) of the curve, a normal vector is given by  f (s,t) = (2/3)(s -1/3, t -1/3 ) For (x,y) on tangent line: (x-s,y-t)  (s -1/3, t -1/3 ) s -1/3 (x-s) + t -1/3 (y-t) = 0 Intercepts at s 1/3 (s 2/3 + t 2/3 ) = s 1/3 L 2/3 and t 1/3 (s 2/3 + t 2/3 ) = t 1/3 L 2/3 Distance between the intercepts is L 2/3 (s 2/3 + t 2/3 ) 1/2 = L

Restatement with p norms ||(x,y)|| p = ( |x| p + |y| p ) 1/p Usual distance is ||(x,y)|| 2 We just saw: For the curve ||(x,y)|| 2/3 = L any tangent line meets the first quadrant in a segment v of length ||v|| 2 = L Generalization: For the curve ||(x,y)|| p = L any tangent line meets the first quadrant in a segment v of length ||v|| q = L where 1/p – 1/q = 1 Call p and q are neoconjugates

Example 1 If p = 2, q = -2. ||(x,y)|| 2 = L on circle of radius L Tangent segment is v = L(sec ,-csc  ) ||v|| -2 = L((sec  ) -2 + (csc  ) -2 ) -1/2 = L

Example 2 If p = 1/2, q = 1. ||(x,y)|| 1/2 = L (restricted to 1 st quadrant) (x 1/2 + y 1/2 ) 2 = L 4xy = (L – x – y) 2 Parabola with axis y = x. ||(x,y)|| 1 = |x| + |y| (taxicab metric) Tangent segments make a string art design with uniformly spaced pins

Restated Ladder Problem Slide a segment of 2-length L around a corner. The corner gap is specified by the vector v = (a,b) The maximum length given by L = ||v|| 2/3

Generalized Ladder Problem Slide a segment of q-length L around a corner. The corner gap is specified by the vector v = (a,b) The maximum length given by L = ||v|| p where 1/p – 1/q = 1 (Need a ladder with a GPS app …)

History of Envelopes In 1940’s and 1950’s, some authors claimed envelopes were standard topic in calculus Nice treatment in Courant’s 1949 Calculus text Some later appearances in advanced calculus and theory of equations books No instance in current calculus books I checked Not included in Thomas (1 st ed.) Still mentioned in context of differential eqns What happened to envelopes?

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