Linnainmaa says the reverse-mode idea came to him on a sunny afternoon in a Copenhagen park in 1970

The origin anecdote for reverse-mode automatic differentiation is recorded, secondhand but in a Tier-1 venue, in the opening prologue of Griewank's 2012 history paper: "Seppo Linnainmaa (Lin76) of Helsinki says the idea came to him on a sunny afternoon in a Copenhagen park in 1970. He used it as a tool for estimating the effects of arithmetic rounding errors on the results of complex expressions."

Three provenance details matter for how this anecdote should be used:

1. It is Linnainmaa's testimony as relayed by Griewank ("says the idea came to him") — the anecdote's authority is Griewank reporting a conversation, not documentary evidence. Griewank elsewhere in the same paper describes phoning Linnainmaa "from Argonne in the late eighties," which is the likely channel.
2. It sits on page 389 (the first page), not page 391 as secondary retellings placed it — the vault's 2026-06-28 capture inherited the wrong page number from a Tier-3/4 chain (Psychology Today UK, a Toronto AI Meetup repost). The quote itself, however, survived that chain verbatim.
3. The anecdote is absent from Schmidhuber's page — Wayback snapshots from 2014–2026 never show it there (see the question-verify-griewank-2012-linnainmaa resolution trail). It belongs to Griewank 2012 alone; retellings that attribute it elsewhere are citing a citation.

The Copenhagen setting fits the documented biography: Griewank's biographical section notes Linnainmaa's early computing work involved the Northern Europe University Computing Center at Copenhagen, which had an IBM 7094.

See claim-linnainmaa-rounding-error-problem for what the idea was for, claim-linnainmaa-thesis-identity for the thesis it landed in, and claim-linnainmaa-priority-not-paternity for why the origin story stayed obscure.