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Binary logarithm - mklabgr - 09-07-2026
Binary Logarithm
The binary logarithm, written $\log_2 n$, is the logarithm to base $2$. It gives the exponent to which $2$ must be raised to obtain $n$:
$\log_2 n = x \Longleftrightarrow 2^x = n$.
For example, $\log_2 8 = 3$ and $\log_2 32 = 5$. It is therefore the inverse of the exponential function $2^x$. Like other logarithms, it satisfies the identities
$\log_2(xy)=\log_2x+\log_2y$,
$\log_2\left(\frac{x}{y}\right)=\log_2x-\log_2y$,
and
$\log_2(x^y)=y\log_2x$.
It can also be calculated using natural logarithms through
$\log_2 n=\frac{\ln n}{\ln 2}$.
Binary logarithms are especially important whenever a process involves repeated doubling or halving. In computer science, binary search repeatedly halves the number of possibilities, so searching among $n$ items requires approximately $\log_2 n$ steps. The number of bits needed to represent a positive integer $n$ is
$\lfloor\log_2 n\rfloor+1$.
For this reason, $\log_2$ occurs throughout information theory, algorithm analysis, binary trees, combinatorics, and data structures. A balanced binary tree containing roughly $n$ elements has height approximately $\log_2 n$, while algorithms such as binary search have complexity $O(\log n)$. In Big-O analysis, the logarithm's base is usually omitted because changing the base only changes the result by a constant factor.
Binary logarithms also appear in bioinformatics, music theory, photography, and tournament scheduling. Their fundamental interpretation is simple: $\log_2 n$ measures how many doublings are needed to reach $n$, or equivalently how many halvings are required to reduce a quantity.
Key Takeaways
- Definition: $\log_2 n$ is the exponent $x$ satisfying $2^x=n$.
- Computing: It naturally describes repeated halving and doubling.
- Bits: $\lfloor\log_2 n\rfloor+1$ gives the number of bits needed to represent a positive integer $n$.
- Algorithms: Binary search has complexity $O(\log n)$.
- Applications: Binary logarithms occur in computer science, information theory, combinatorics, bioinformatics, music theory, and related fields.
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