WHO INVENTED BACKPROPAGATION?

Summary from the TLDR Web Dev newsletter

Efficient backpropagation (used in modern neural networks) was first published by Seppo Linnainmaa in 1970, who built on precursor work by Henry J. Kelley in 1960. The minimization of errors through gradient descent in neural networks using dynamic programming and the chain rule has been discussed since the early 1960s. Despite its early origins, it took four decades for backpropagation to gain widespread acceptance.