Although Irving Fisher did do his PhD in the US,
his relevant works on capital and interest were
written after his postdoctoral year at the University
of Berlin. His remark in The Nature of Capital and
Income (1906, p. 205) that "In Germany and some
other countries, such appraisement of forests is
now worked out with considerable precision"
suggests that he had heard of the existence of
a German literature on forestry (perhaps while
at Berlin, or perhaps from speaking with faculty
of the Yale School of Forestry), but had not
studied that literature himself. Fisher extensively
cited German-language economic literature that
he had read: see The Nature of Capital and Income
(1906, pp. 351-356) quoting German definitions
of income and The Rate of Interest (1907, pp.
423-425) surveying German literature on statistics
of interest rates.

As to the extent of Paul Krugman's acknowledgement
of Masahisa Fujita, the two names do appear
together on the title page of Fujita, Krugman, and
Venables, The Spatial Economy (MIT Press, 1999).

Robert Dimand