Google's version includes coöperation, but it has negligible usage: https://books.google.com/ngrams/graph?content=cooperation%2Cco-operation%2Cco%C3%B6peration&

Google's version includes coöperation, but it has negligible usage: https://books.google.com/ngrams/graph?content=cooperation%2Cco-operation%2Cco%C3%B6peration&year_start=1800&year_end=2008&corpus=15 https://books.google.com/ngrams/graph?content=cooperation%2C... On the other hand it's possible OCR is missing some of those ¨ and screwing up the data.