Definition of "coöperate"

The best progress is made by combining the practice of coöperation with the science of coöperation. They are one and inseparable to the fullest benefit. But the real way to coöperate is to coöperate.

1891, Herbert Myrick, chapter 1, in How to Coöperate: The full fruits of labor to producer, honest value to consumer, just return to capital, prosperity to all. A manual for coöperators‎, New York: Orange Judd, page 10

Other bodies of Federal troops had advanced into Virginia to coöperate with the main force on the Peninsula. [...] This would completely insure the communications of his army from interruption; and it was no doubt expected that Generals Fremont and Banks would coöperate in the movement also.

2007, John Esten Cooke, A Life of Gen. Robert E. Lee‎, page 40

Zeldin imagined Mueller taking such a step to pressure Manafort to coöperate.

2017 October, John Cassidy, “Robert Mueller Sends a Message: He’s Deadly Serious”, in The New Yorker‎