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residual vector

\mathbf r = \mathbf b - A\mathbf x — the gap between actual and predicted output.

For a candidate \mathbf x, the residual vector is \mathbf r(\mathbf x) = \mathbf b-A\mathbf x. The least-squares problem minimizes \lVert\mathbf r(\mathbf x)\rVert^2=\sum r_i^2, the sum of squared residuals (SSE).

See also

least-squares solution R-squared
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