# Undefined control sequence error for sum expression in equation

The below sum expression inside the equation throws an undefined control sequence error.

$$\left\|x\left[t\right]-\textbf{1} F(X)\right\|^2 = \sum_{i = 1}^{n} \lambda_i^2t \lvert x \left[ 0 \right] \rvert^2$$

Error:

Undefined control sequence.

|^2 = \sum_{i = 1}^{n} \lambda_i^2t \lvert
x \left[ 0 \right] \rvert^2

My guess is I am doing something wrong with the \lvert or \lambda. Don't know exactly. Any suggestion on how to solve this issue will be very helpful.

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Welcome to TeX.SE. It is always best to compose a fully compilable MWE that illustrates the problem including the \documentclass and the appropriate packages so that those trying to help don't have to recreate it. – Peter Grill May 21 '12 at 17:36

$$\left\|x\left[t\right]-\textbf{1} F(X)\right\|^2 = \sum_{i = 1}^{n} \lambda_i^2t \lvert x \left[ 0 \right] \rvert^2$$