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  "body": "\n## Summary\n\nThe Riesz projector of an operator $T$ for an isolated part of its spectrum is a projection.\nIt is obtained by integrating the resolvent of $T$ around that part.\nIt splits the space into two closed invariant subspaces on which $T$ has the separated pieces of the spectrum.\nIt generalises the projection onto a generalised eigenspace of a matrix along the other generalised eigenspaces.\n\n## Explanations\n\n### L1 human\n\nFor a matrix, one can split the space into the generalised eigenspace of one eigenvalue and the sum of the others.\nThe Riesz projector does this for operators, for any part of the spectrum that is separated from the rest by a gap.\nIt is computed by a contour integral around that part, as in Cauchy's integral formula.\nFor the diagonal matrix with entries $1$ and $2$, the Riesz projector for $\\set{1}$ keeps the first coordinate and kills the second.\n",
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