Lambda (λ) in HTML
Lambda is the eleventh Greek letter, written Λ and λ. The capital is at U+039B and the small letter at U+03BB.
At a glance
- Character
- Λ λ
- Named entity
- Λ λ
- Decimal reference
- Λ λ
- Hex reference
- Λ λ
- Unicode code point
- U+039B, U+03BB
- Unicode name
- GREEK CAPITAL LETTER LAMBDA, GREEK SMALL LETTER LAMBDA
- In the entity reference
- Greek
In detail
In physics the small lambda is wavelength, and it appears in the relationship connecting the speed of a wave to its frequency. In radioactivity and in exponential decay generally it is the decay constant, which is the reciprocal of the mean lifetime, and in queueing theory and Poisson processes it is the arrival rate. The three uses share a flavour: lambda is the parameter that sets a scale.
The programming use comes from lambda calculus, a formal system for expressing computation in terms of function abstraction and application. That is why an anonymous function is called a lambda in so many languages, and why a few of them use the letter itself in their syntax rather than a keyword.
In machine learning lambda is conventionally the regularisation strength, the weight applied to a penalty term that discourages large coefficients. It is a hyperparameter rather than something learned, which means it is chosen by validation, and reporting a model without reporting its lambda leaves the result unreproducible.
The capital lambda is drawn as a bare triangle without its base, close enough to a Latin A with the crossbar removed that it reads as an ornament in display type. It appears in physics as the cosmological constant and in a few notations for logical conjunction, but it is not common enough that a reader will guess your meaning from context.
Examples
<p>c = λν, so λ = c / ν</p>
<p>Green light is around λ = 550 nm.</p>ν is the Greek small letter nu, conventionally used for frequency.
const lambda = 0.01;
const loss = mse(predictions, targets) + lambda * l2Norm(weights);Things that catch people out
- The decay constant and the mean lifetime are reciprocals, not the same number. Halving one doubles the other.
- A reported model without its regularisation strength cannot be reproduced, since the hyperparameter changes the fit entirely.
- Writing the Greek letter in a variable name or a file path makes the identifier hard to type and hard to search for.
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Related HTML entities
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The Greek letter mu at U+039C and U+03BC, its entity forms, the separate micro sign it duplicates, and its meanings in statistics and mechanics.
Theta (θ) in HTML
The Greek letter theta at U+0398 and U+03B8, its entity forms, its role as the default angle variable, and what big theta asserts about an algorithm.
Omega (ω) in HTML
The Greek letter omega at U+03A9 and U+03C9, its entity forms, the separate ohm sign that normalises onto it, and its use for angular frequency.
Pi (π) in HTML
The Greek letter pi at U+03A0 and U+03C0, its entity forms, the capital pi that is not the product operator, and how to write pi in code correctly.
Sigma (σ) in HTML
The Greek letter sigma at U+03A3 and U+03C3, its entity forms, the summation operator it is confused with, and the final sigma used at the end of a word.
Phi (φ) in HTML
The Greek letter phi at U+03A6 and U+03C6, its entity forms, the loopy and straight glyph variants, and the golden ratio it conventionally names.
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