Abstract
We show that in O
(D)
invariant matrix theories containing a large number
D
of complex or Hermitian matrices, one can define a
D \to \infty
limit for which the sum over planar diagrams truncates to a tractable, yet non-trivial, sum over melon diagrams. In particular, results obtained recently in SYK and tensor models can be generalized to traditional, string-inspired matrix quantum mechanical models of black holes.