  In this paper we investigate the problem of simultaneously allocating orders
and mobile storage racks to static pickers. Here storage racks are allocated to
pickers to enable them to pick all of the products for the orders that have
been allocated to them. Problems of the type considered here arise in
facilities operating as robotic mobile fulfilment systems.
  We present a formulation of the problem of allocating orders and racks to
pickers as an integer program and discuss the complexity of the problem. We
present two heuristics (matheuristics) for the problem, one using partial
integer optimisation, that are directly based upon our formulation.
  We also consider the problem of how to sequence the racks for presentation at
each individual picker and formulate this problem as an integer program. We
prove that, subject to certain conditions being satisfied, a feasible rack
sequence for all orders can be produced by focusing on just a subset of the
orders to be dealt with by the picker.
  Computational results are presented, both for order and rack allocation, and
for rack sequencing, for randomly generated test problems (that are made
publicly available) involving up to 500 products, 150 orders, 150 racks and 10
pickers.
