- #1
QuietMind
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Homework Statement
Prove that if p(x) has even degree with positive leading coefficient, and ## p(x) - p''(x) \geq 0 ## for all real x, then $$ p(x) \geq 0$$ for all real x
Homework Equations
N/A
Problem is from Art and Craft of Problem Solving, as an exercise left to the reader following a partial solution in the text. The text goes over a somewhat similar example: when we have a polynomial q(x) such that ##q(x) - q'(x) \geq 0##. The text showed us that we can determine that q(x) must be of even degree with leading coefficient positive.
The Attempt at a Solution
I have thought about a few different possible strategies:
1) Show that p''(x) is always nonnegative. I'm not sure how to go about this strategy, or if this conjecture is even true.
2) Show that p(x) either has no zeroes, or only has zeroes that barely touch the x-axis (therefore it lies on the upper half plane). If p(x) does have a zero at some point, say ##x_0## then ##p(x_0) - p''(x_0) \geq 0##. If ##p(x_0) = 0## then ##p''(x_0) = 0##. This is a necessary but not sufficient condition for an inflection point at ##x_0##. This seems significant but how does this help me?