Wikipedia:Reference desk/Archives/Mathematics/2012 October 23

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October 23

Generators of a ring of polys with a particular factor.

What are the generators of the ring of real polynomials divisible by $x^{2}-4x+5$? Widener (talk) 03:09, 23 October 2012 (UTC)

I take it you are allowing your rings to be non-unital. I think the polynomials divisble by $x^{2}-4x+5\,$ will be generated by $x^{2}-4x+5\,$ and $x(x^{2}-4x+5)\,$, since

$x^{n+2}(x^{2}-4x+5)=((x^{2}-4x+5)x^{n}+4x^{n+1}-5x^{n})(x^{2}-4x+5)$

Obviously other pairs of generators can be chosen, such as $(x+1)(x^{2}-4x+5)\,$ and $(2x+1)(x^{2}-4x+5)\,$. Gandalf61 (talk) 08:39, 24 October 2012 (UTC)

A simple calculus question

If we have ${\partial ^{2}f}/{\partial x^{2}}$ and replace $x=k.t$ ,k is a constant, what is the denominator of fraction, ${k\partial t^{2}}$ or ${k^{2}\partial t^{2}}$  ? — Preceding unsigned comment added by Re444 (talk • contribs) 12:37, 23 October 2012 (UTC)

The question doesn't entirely make sense: a partial derivative is not a fraction (and hence has no denominator), and in any event substituting a variable with respect to which a partial derivative is taken cannot be done without simultaneously specifying what the other independent variables (that are being held constant) are replaced by. However, if we look at the standard derivative instead, we have that if x = kt with k constant throughout the domain of interest, then

${\frac {d^{2}f}{dx^{2}}}={\frac {1}{k^{2}}}\cdot {\frac {d^{2}f}{dt^{2}}}.$

— Quondum 13:10, 23 October 2012 (UTC)