|WikiProject Mathematics||(Rated Start-class, Mid-importance)|
What the hell is x times gradient of f(x) supposed to mean, dot product?
- It means that for a vector function f(x) that is homogenous of degree k, the dot production of a vector x and the gradient of f(x) evaluated at x will equal k * f(x). CodeLabMaster 12:12, 05 August 2007 (UTC)
- Yes, as can be seen from the furmula under that one. I've added the dot and changed vector symbols to bold. mazi 18:04, 22 February 2006 (UTC)
Not all homogeneous functions are differentiable
The article, before I changed it a moment ago, implied that all homogeneous functions are differentiable. Here's a counterexample: f: R^2 --> R, k=1, with f(x,y)= either x (if xy>0) or 0 (otherwise). --Steve 03:23, 6 August 2007 (UTC)
Is the derivative theorem correct?
According to planetmath , the theorem about derivatives is not correct unless we replace "homogeneous" by "positive homogeneous" throughout. Their counterexample is wrong (I just submitted a correction on the site), but could that claim be correct? Does anyone have a reference, or a proof, or a proper counterexample?
Update: The person maintaining that planetmath page responded to my correction by taking away the counterexample but keeping the claim. Again, a reference, proof, or proper counterexample is needed to resolve this. --Steve 15:51, 5 October 2007 (UTC)
The result is correct for functions which are homogeneous of degree . I've added the elementary proof of this result to the page (and merged "Other properties" with "Euler's theorem" as the proofs are very similar). Is the planetmath contributor worried about ? Clearly the definition of homogeneous of degree for has to be modified so that the condition holds for all , and I've just changed this too. Mark (talk) 16:31, 11 February 2008 (UTC)
Notation such as
can be confusing: Are we differentiating the expression with respect to the first component of or do we mean the partial derivative of with respect to its first argument evaluated at the point ?
It's therefore better to write
now it's clear that are the arguments of and we are differentiating with respect to the first argument of evaluated at the point .
- I agree. Sorry about my incorrect edit to that effect earlier, thanks for reverting :-) --Steve (talk) 18:00, 11 February 2008 (UTC)
- Keep in mind the Euler lived in the 18th century and wrote mostly in Latin so not really a good reference for a modern audience. It might be worth adding the original work as a historical reference though.--RDBury (talk) 21:44, 18 April 2010 (UTC)
restriction of k for nth degree homogeneity?
What are the restrictions on k? Must k be contained within the domain of the vector space, reals, or what?
- For arbitrary fields, k should be an integer. For the reals, it makes sense to define this notion also for real numbers k. For example, the square root is homogeneous with k=1/2.
- But it seems to me that usually k is an integer. --Aleph4 (talk) 16:49, 16 May 2011 (UTC)
Note on change http://en.wikipedia.org/w/index.php?title=Homogeneous_function&diff=529079679&oldid=520543743
The Phi used in the equations, that is /varphi inside a <math> tag, was rendered differently in my browser (mobile Safari) than the &phi used in the descriptions below (see Phi#Computing) -- so I switched the descriptions to use the clumsier but definitely-consistent <math> tag construction. 18.104.22.168 (talk) 06:26, 21 December 2012 (UTC)