Names of 661
| Nominal | 661 |
| Cardinal | six hundred sixty-one |
| Ordinal | 661st |
| This number as US currency | six hundred sixty-one dollars |
| Is 661 an odd number? | 661 is an odd number |
| Is 661 an even number? | 661 is NOT an even number |
| Is 661 a palindrome? | 661 is NOT a palindrome number |
| Is 661 a triangle number? | 661 is NOT a triangle number |
661 in other Bases
| Base 2 (Binary) | 10100101012 |
| Base 3 (Ternary) | 2201113 |
| Base 4 (Quaternary) | 221114 |
| Base 5 (Quinary) | 101215 |
| Base 6 (Senary) | 30216 |
| Base 8 (Octal) | 12258 |
| Base 12 (Duodecimal) | 47112 |
| Base 16 (Hexadecimal) | 29516 |
| Base 20 (Vigesimal) | 1D120 |
Primes and Factors tests for 661
| Is 661 a Prime Number? | 661 is a prime |
| Is 661 a perfect number? | 661 is NOT a perfect number |
| Is 661 a Mersenne Prime? | 661 is NOT a Mersenne prime |
| Is 661 a Fermat Prime? | 661 is NOT a Fermat prime |
| Is 661 a Fibonacci Prime? | 661 is NOT a Fibonacci prime |
| Is 661 a Partition Prime? | 661 is NOT a Partition prime |
| Is 661 a Pell Prime? | 661 is NOT a Pell prime |
| Prime Factors | Prime factors of 661 |
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661 in other Numerals
| 661 in Roman Numeral | DCLXI |
| 661 in Chinese Numeral | 陸佰陸拾壹 |
| 661 in Egyptian Numeral | 𓍧𓎋𓏺 |
| 661 in Babylonian Numeral |   |
Roots and stuff for 661
| Is 661 a square number? | 661 is NOT a square number |
| Is 661 a cube number? | 661 is NOT a cube number |
| Square root of the number 661 | 25.709920264365 |
| Cube root of the number 661 | 8.7109827387101 |
Recreational math for 661
| Number 661 reversed | 166 |
| Sum of the digits | 13 |
| No of digits | 3 |