  Plane partitions have been widely studied in Mathematics since MacMahon. See,
for example, the works by Andrews, Macdonald, Stanley, Sagan and Krattenthaler.
The Schur process approach, introduced by Okounkov and Reshetikhin, and further
developed by Borodin, Corwin, Corteel, Savelief and Vuleti\'c, has been proved
to be a powerful tool in the study of various kinds of plane partitions. The
exact enumerations of ordinary plane partitions, shifted plane partitions and
cylindric partitions could be derived from two summation formulas for Schur
processes, namely, the open summation formula and the cylindric summation
formula.
  In this paper, we establish a new summation formula for Schur processes,
called the complete summation formula. As an application, we obtain the
generating function and the asymptotic formula for the number of doubled
shifted plane partitions, which can be viewed as plane partitions `shifted at
the two sides'. We prove that the order of the asymptotic formula depends only
on the diagonal width of the doubled shifted plane partition, not on the
profile (the skew zone) itself. By using the same methods, the generating
function and the asymptotic formula for the number of symmetric cylindric
partitions are also derived.
