Certain aspects of Street's formal theory of monads in 2-categories are
extended to multimonoidal monads in symmetric strict monoidal
2-categories. Namely, any symmetric strict monoidal 2-category M admits a
symmetric strict monoidal 2-category of pseudomonoids, monoidal 1-cells
and monoidal 2-cells in M. Dually, there is a symmetric strict monoidal
2-category of pseudomonoids, opmonoidal 1-cells and opmonoidal 2-cells in
M. Extending a construction due to Aguiar and Mahajan for M = Cat, we may
apply the first construction p-times and the second one q-times (in any
order). It yields a 2-category M_{pq}. A 0-cell therein is an object A of
M together with p+q compatible pseudomonoid structures; it is termed a
(p+q)-oidal object in M. A monad in M_{pq} is called a (p,q)-oidal monad
in M; it is a monad t on A in M together with p monoidal, and q
opmonoidal structures in a compatible way. If M has monoidal
Eilenberg-Moore construction, and certain (Linton type) stable
coequalizers exist, then a (p+q)-oidal structure on the Eilenberg-Moore
object A^t of a (p,q)-oidal monad (A,t) is shown to arise via a symmetric
strict monoidal double functor to Ehresmann's double category Sqr(M) of
squares in M, from the double category of monads in Sqr(M) in the sense
of Fiore, Gambino and Kock. While q ones of the pseudomonoid structures
of A^t are lifted along the `forgetful' 1-cell A^t -> A, the other p ones
are lifted along its left adjoint. In the particular example when M is an
appropriate 2-subcategory of Cat, this yields a conceptually different
proof of some recent results due to Aguiar, Haim and Lopez Franco.
