LeCun's own account dates the rigorous, Lagrangian (duality-adjacent) derivation of backpropagation to his 1988 paper, not the 1985 Cognitiva paper

yann-lecunpaul-werbosarthur-brysonbackpropagationpriority-vs-paternityhistory-of-ml

On the same research page as claim-lecun-1985-virtual-target-values-not-gradients, in the next section ("Learning Theory, Learning Algorithms"), LeCun writes: "During my PostDoc in Toronto, I extended and published my thesis work on the theoretical foundation of backpropagation, and showed that back-propagation and several of its generalizations could be derived rigorously using Lagrange functions[LeCun, 1988]." Lagrange-multiplier derivations are the same mathematical family claim-recht-adjoints-equivalence-not-transmission documents as the generic control-theory/duality route into backpropagation — multipliers running backward as a dual, adjoint system. LeCun's own account places this rigorous derivation in his 1988 paper, "A Theoretical Framework for Back-Propagation" (Proceedings of the 1988 Connectionist Models Summer School, Carnegie Mellon, pp. 21–28), not in the 1985 Cognitiva paper, which by his own description used a different, non-gradient mechanism ("virtual target values").

A Tier-4 pointer offers weak corroboration of the same date, not independent confirmation of it: the IEEE Milestones program's ETHW page on "Convolutional Neural Networks, 1989" states "he derived a theoretical framework of the backpropagation algorithm and showed its connection to the literature of control theory," footnoting a 1987 AT&T Bell Laboratories technical report of the same title as the 1988 paper above. As a committee-written plaque-citation wiki page, this is cited only as a pointer to the same later document, per the sourcing floor's treatment of tertiary aggregators — not as a second, independently-reported fact.

Read together with claim-lecun-1985-virtual-target-values-not-gradients, this suggests claim-werbos-2004-object-nets-patent-names-lecun-against-1994-book's attribution of a "duality theory from control theory" derivation to "LeCun (1985)" more plausibly belongs to LeCun's 1988 paper — a compression of three years and two publications into one citation, in a document written by Werbos to argue his own priority case. See question-does-lecuns-1985-derivation-match-werboss-duality-theory-characterization for the still-open textual question of what the 1985 paper itself contains.