Equidigital number

In number theory, an equidigital number is a natural number in a given number base that has the same number of digits as the number of digits in its prime factorization in the given number base, including exponents but excluding exponents equal to 1.[1] For example, in base 10, 1, 2, 3, 5, 7, and 10 (2 · 5) are equidigital numbers (sequence A046758 in the OEIS). All prime numbers are equidigital numbers in any base.

Demonstration, with Cuisenaire rods, that the composite number 10 is equidigital: 10 has two digits, and 2 · 5 has two digits (1 is excluded)

A number that is either equidigital or frugal is said to be economical.

Mathematical definition

Let ${\displaystyle b>1}$ be a number base, and let ${\displaystyle K_{b}(n)=\lfloor \log _{b}{n}\rfloor +1}$ be the number of digits in a natural number ${\displaystyle n}$ for base ${\displaystyle b}$. A natural number ${\displaystyle n}$ has the integer factorisation

${\displaystyle n=\prod _{\stackrel {p\mid n}{p{\text{ prime}}}}p^{v_{p}(n)}}$

and is an equidigital number in base ${\displaystyle b}$ if

${\displaystyle K_{b}(n)=\sum _{\stackrel {p\mid n}{p{\text{ prime}}}}K_{b}(p)+\sum _{\stackrel {p^{2}\mid n}{p{\text{ prime}}}}K_{b}(v_{p}(n))}$

where ${\displaystyle v_{p}(n)}$ is the p-adic valuation of ${\displaystyle n}$.

See also

• Extravagant number
• Frugal number
• Smith number

Notes

1. Darling, David J. (2004). The universal book of mathematics: from Abracadabra to Zeno's paradoxes. John Wiley & Sons. p. 102. ISBN 978-0-471-27047-8.

References

• R.G.E. Pinch (1998), Economical Numbers.