James Mercer (mathematician)

{{Use dmy dates|date=April 2022}}

James Mercer FRS{{Cite journal | last1 = Hobson | first1 = E. W. | title = James Mercer. 1883-1932 | doi = 10.1098/rsbm.1933.0016 | journal = Obituary Notices of Fellows of the Royal Society | volume = 1 | issue = 2 | pages = 164–165 | year = 1933 | author1-link = E. W. Hobson }} (15 January 1883 – 21 February 1932) was a mathematician, born in Bootle, close to Liverpool, England.{{MacTutor Biography|id=Mercer|title=James Mercer}}

He was educated at University of Manchester, and then University of Cambridge. He became a Fellow, saw active service at the Battle of Jutland in World War I and, after decades of ill health, died in London.

He proved Mercer's theorem, which states that positive-definite kernels can be expressed as a dot product in a high-dimensional space. This theorem is the basis of the kernel trick (applied by Aizerman), which allows linear algorithms to be easily converted into non-linear algorithms.