**225** (**two hundred [and] twenty-five**) is the natural number following 224 and preceding 226.

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Cardinal | two hundred twenty-five | |||

Ordinal | 225th (two hundred twenty-fifth) | |||

Factorization | 3^{2}× 5^{2} | |||

Prime | no | |||

Greek numeral | ΣΚΕ´ | |||

Roman numeral | CCXXV | |||

Binary | 11100001_{2} | |||

Ternary | 22100_{3} | |||

Quaternary | 3201_{4} | |||

Quinary | 1400_{5} | |||

Senary | 1013_{6} | |||

Octal | 341_{8} | |||

Duodecimal | 169_{12} | |||

Hexadecimal | E1_{16} | |||

Vigesimal | B5_{20} | |||

Base 36 | 69_{36} |

225 is the smallest number that is a polygonal number in five different ways.^{[1]} It is a square number (225 = 15^{2}),^{[2]}
an octagonal number,^{[3]} and a squared triangular number (225 = (1 + 2 + 3 + 4 + 5)^{2} = 1^{3} + 2^{3} + 3^{3} + 4^{3} + 5^{3}) .^{[4]}

As the square of a double factorial, 225 = 5!!^{2} counts the number of permutations of six items in which all cycles have even length, or the number of permutations in which all cycles have odd length.^{[5]} And as one of the Stirling numbers of the first kind, it counts the number of permutations of six items with exactly three cycles.^{[6]}

225 is a highly composite odd number, meaning that it has more divisors than any smaller odd numbers.^{[7]} After 1 and 9, 225 is the third smallest number *n* for which *σ*(*φ*(*n*)) = *φ*(*��*(*n*)), where *σ* is the sum of divisors function and *φ* is Euler's totient function.^{[8]} 225 is a refactorable number.^{[9]}

225 is the smallest square number to have one of every digit in some number base (225 is 3201 in base 4) ^{[10]}

## In other fields

- The years 225 and 225 BC
- .225 Winchester, firearm cartridge

## References

**^**Sloane, N. J. A. (ed.). "Sequence A063778 (a(n) = the least integer that is polygonal in exactly n ways)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation.**^**Sloane, N. J. A. (ed.). "Sequence A000290 (The squares)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation.**^**Sloane, N. J. A. (ed.). "Sequence A000567 (Octagonal numbers)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation.**^**Sloane, N. J. A. (ed.). "Sequence A000537 (Sum of first n cubes; or n-th triangular number squared)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation.**^**Sloane, N. J. A. (ed.). "Sequence A001818 (Squares of double factorials)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation.**^**Sloane, N. J. A. (ed.). "Sequence A000399 (Unsigned Stirling numbers of first kind s(n,3))".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation.**^**Sloane, N. J. A. (ed.). "Sequence A053624 (Highly composite odd numbers)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation.**^**Sloane, N. J. A. (ed.). "Sequence A033632 (Numbers n such that sigma(phi(n)) = phi(sigma(n)))".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation.**^**"Sloane's A033950 : Refactorable numbers".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation. 2016-04-18. Retrieved 2016-04-18.**^**Sloane, N. J. A. (ed.). "Sequence A061845 (Numbers which have one of every digit in some base)".*The On-Line Encyclopedia of Integer Sequences*. OEIS Foundation.

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