Lambda calculus
Origin of the λ symbol

There is some uncertainty over the reason for Church's use of the Greek letter lambda (λ) as the notation for function-abstraction in the lambda calculus, perhaps in part due to conflicting explanations by Church himself. According to Cardone and Hindley (2006):

By the way, why did Church choose the notation “λ”? In he stated clearly that it came from the notation “ x ^ {\displaystyle {\hat {x}}} ” used for class-abstraction by Whitehead and Russell, by first modifying “ x ^ {\displaystyle {\hat {x}}} ” to “ ∧ x {\displaystyle \land x} ” to distinguish function-abstraction from class-abstraction, and then changing “ ∧ {\displaystyle \land } ” to “λ” for ease of printing.

This origin was also reported in. On the other hand, in his later years Church told two enquirers that the choice was more accidental: a symbol was needed and λ just happened to be chosen.

Dana Scott has also addressed this question in various public lectures. Scott recounts that he once posed a question about the origin of the lambda symbol to Church's former student and son-in-law John W. Addison Jr., who then wrote his father-in-law a postcard:

Dear Professor Church,

Russell had the iota operator, Hilbert had the epsilon operator. Why did you choose lambda for your operator?

According to Scott, Church's entire response consisted of returning the postcard with the following annotation: " eeny, meeny, miny, moe ".
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