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Some resources such as https://online.stat.psu.edu/stat508/book/export/html/696, give the following for the quadratic discriminant function; $$ln(\pi_k)-\frac{1}{2}(x-\mu_k)^T\Sigma_k^{-1}(x-\mu_k)-ln(|\Sigma_k|^{1/2})$$

whereas 'An Introduction to Statistical Learning' omits this last term.

Why do they omit the last term? It depends on k so surely it should be relevant to the maximisation?

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  • $\begingroup$ The resource in arxiv.org/abs/1906.02590 includes that term. It should only vanish in the case of LDA, when all covariance matrices are equal $\endgroup$
    – PedroSebe
    Commented Oct 6, 2020 at 10:47

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This is listed as errata for an early edition of the book (1st edition prior to the 4th printing) http://faculty.marshall.usc.edu/gareth-james/ISL/errata.html

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