  This paper studies extension groups between certain Weyl modules for the
algebraic group GL_n over the integers. Main results include: (1) A complete
determination of Ext groups between Weyl modules whose highest weights differ
by a single root and (2) Determination of Ext^1 between an exterior power of
the defining representation and any Weyl module. The significance of these
results for modular representation theory of GL_n is discussed in several
Remarks. Notably the first result leads to a calculation of Ext groups between
neighboring Weyl modules for GL_n and also recovers the GL_n case of a recent
result of Andersen. Some generalities about Ext groups between Weyl modules and
a brief overview of known results about these groups are also included.
