- Defining an
exact (analytic) solution
in general relativity
(i.e., for physical systems)
is a bit tricky and so is counting exact solutions
since many are special cases of others.
- By one expert's count, there are,
circa 2022, only
6 top-level
(i.e., general and important) exact solutions in general relativity (see M.A.H. MacCallum, 2013,
Exact solutions of Einstein's equations).
There are many special cases of the 6 top-level exact solutions in general relativity.

- Long ago in the 1990s,
yours truly vaguely recalls a
Russian speaker who was
introduced as having discovered 2
of the
6 top-level
exact solutions in general relativity.
- To the knowledge of yours truly,
Albert Einstein (1879--1955) himself
found only one exact solution---the
Einstein universe
in field of
cosmology---which
is taken up in IAL 30: Cosmology.
And it is
**NOT**one of the 6 top-level exact solutions in general relativity. The Einstein universe is a special case of the Friedmann-Lemaitre-Robertson-Walker solution in the count of M.A.H. MacCallum (2013). - Finding new exact solutions
in general relativity is always hard work
and it gets harder with time since only harder ones are left to be found.

Exact solutions in general relativity explicated:

Image link: Itself.

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