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I am currently studying linear programming and am attempting to solve:

minimize $c^Tx$

subject to $\sum_{i=1}^{n}x_i=0$, and $\sum_{i=1}^{n}x_i^2 = 1$.

From the second constraint I know that: $-1\le x_i \le 1$, which can be split up into two separate inequalities. This in turn implies that $\sum_{i=1}^n|x_i| \le n$. I am unsure what to do from here, and whether or not the last inequality is useful in solving the problem.

Any help would be greatly appreciated, thanks.

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    $\begingroup$ The second constraint involves a quadratic expression and hence is not linear. $\endgroup$ – Rob Pratt Nov 14 at 2:26
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As Rob points out, this is isn't a linear program. However, you can still solve it.

Start by thinking about minimizing $c^t x$ over the unit sphere. If you draw some pictures in $2D$, you will guess an answer. You can verify it using the Cauchy-Schwartz inequality. Now extend to $\mathbb{R}^n$.

Now you can extend your solution to the problem you gave. Does the component of $c$ orthogonal to the plane defined by $\{ \sum x_i = 0 \}$ matter?

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