Christos Dimitriou Papakyriakopoulos, commonly known as "Papa" (Χρήστος Δημητρίου Παπακυριακόπουλος in Greek; born in Chalandri, Athens, Greece, on June 29, 1914, died on June 29, 1976) was a Greek mathematician specializing in geometric topology. He worked in isolation at Athens University being awarded a Ph.D on the recommendation of Carathéodory. In 1948 he was invited by Ralph Fox to come as his guest at the Princeton mathematics department. Fox had been impressed by a letter from Papakyriakopoulos which purported to prove Dehn's lemma. The proof, as it turned out was faulty, but Fox's sponsorship would continue for many years and enabled Papakyriakopoulos to work on his mathematics without concern of financial support.

Papakyriakopoulos is best known for his proofs of Dehn's lemma, the loop theorem, and the sphere theorem, three foundational results for the study of 3-manifolds. In honor of this work, he was awarded the first Veblen Prize in Geometry in 1964. From the early sixties on, he mostly worked on the Poincaré conjecture.

The following unusual limerick was composed by John Milnor, shortly after learning of several graduate students' frustration at completing a project where the work of every Princeton mathematics faculty member was to be summarized in a limerick:

The perfidious lemma of Dehn

Was every topologist's bane

      'Til Christos Papa-

      kyriakopou-

los proved it without any strain.

The phrase "without any strain" is not meant to indicate that Papa did not expend much energy in his efforts. Rather, it refers to Papa's "tower construction", which quite nicely circumvents much of the difficulty in the cut - and - paste efforts that preceded Papa's proof.

He sympathized with leftist politics. When he went to live in the USA, the Greek authorities reported him to the American authorities as a "communist".

He was a reclusive character, spending most of his time in his office listening to his beloved Wagner. Legend has it that in the US he lived for 25 years in the same hotel room he used when he first arrived in the country, all of his belongings inside his original luggage.

He died of cancer at age 62 in Princeton, New Jersey.