I'm searching the wheres and whys about the integral notation for co/ends. Who was the first to adopt it? Can you give me a precise pointer or tell me the whole story about it? Was s/he motivated by the Fubini rule only, or there is another additional reason?
Thanks a lot!
Best Answer
From Ross Street's An Australian conspectus of higher categories:
The Day-Kelly paper referred to is
B. Day and G.M. Kelly, Enriched functor categories, Reports of the Midwest Category Seminar, III, Springer, Berlin, 1969, pp. 178–191.
whose introduction states
The Yoneda paper referred to is
Yoneda, N., On Ext and exact sequences. Jour. Fac. Sci. Univ. Tokyo 8 (1960), 507 - 576.
where we find written on page 546 (thanks to Francesco for the page reference)
where his "integration" is our "coend", and dually.