Lambda symbol λ

• Wavelength: visible light has wavelengths λ from roughly 380 to 750 nm, and c = λf links wavelength, frequency and the speed of light.
• Eigenvalues: in linear algebra, Av = λv.
• Decay constant: in radioactive decay, N(t) = N₀e^(−λt).
• Rates: in statistics, λ is the rate of a Poisson distribution, the average number of events per interval.
• Lambda calculus: Alonzo Church’s mathematical model of computation, which gave programming its “lambda functions.”

The capital Λ is less common in formulas. It stands for the cosmological constant in physics and often for the diagonal matrix of eigenvalues in linear algebra.

An anonymous function is often called a lambda, after lambda calculus:

• Python: square = lambda x: x * x
• JavaScript: arrow functions such as x => x * x play the same role.
• Java and C#: x -> x * x and x => x * x.
• Haskell: \x -> x * x; the backslash was chosen because it resembles λ.

In code you type the keyword or the arrow, not the Greek letter. λ itself appears in documentation, slides and diagrams.

Look-alikes: Λ (capital lambda) looks like an upside-down V and is easy to confuse with ∧ (logical and). ƛ (lambda with stroke) is a Latin letter used in phonetic writing for some Indigenous languages of North America, not a Greek letter.

• Word: in an equation (Alt+=), type \lambda and press space; \Lambda gives the capital.
• LaTeX: \lambda and \Lambda.
• Mac: open the Character Viewer with Control+Command+Space and search “lambda.”
• iPhone and Android: add the Greek keyboard; λ is where L is on a US keyboard.
• HTML: paste it directly into UTF-8 pages, or use the named entity from the table above.