661
domain: N
Properties
Digital Properties
- Digit Count
- 3
- Digit Sum
- 13
- Digital Root
- 4
- Palindromic Number
- no
- Repdigit
- no
- Automorphic
- no
- Kaprekar Number
- no
- Multiplicative Persistence
- 3
Divisibility
- Divisor Count
- 2
- Divisor Sum
- 662
- Proper Divisor Sum (Aliquot Sum)
- 1
- Abundant Number
- no
- Perfect Number
- no
- Deficient Number
- yes
- Highly Composite
- no
- Weird Number
- no
- Untouchable Number
- no
- Primitive Abundant
- no
Derived Values
- Euler's Totient
- 660
- Möbius Function
- -1
- Radical
- 661
- Omega Function (Ω)
- 1
- Little Omega Function (ω)
- 1
Special
- Factorial
- no
- Catalan Number
- no
- Bell Number
- no
- Motzkin Number
- no
- Primorial
- no
Figurate Numbers
- Fibonacci Number
- no
- Triangular Number
- no
- Perfect Square
- no
- Perfect Cube
- no
- Pentagonal Number
- no
- Hexagonal Number
- no
- Lucas Number
- no
- Tetrahedral Number
- no
- Pell Number
- no
- Tribonacci Number
- no
- Pronic Number
- no
Recreational
- Happy Number
- no
- Harshad Number
- no
- Narcissistic Number
- no
- Collatz Steps
- 113
- Smith Number
- no
Primality
- Prime
- yes
- Composite Number
- no
- Semiprime
- no
- Squarefree Number
- yes
- Prime Power
- yes
- Prime Factorization
- no
- Twin Prime
- yes
- Mersenne Prime
- no
- Sophie Germain Prime
- no
- Safe Prime
- no
- Powerful Number
- no
- Achilles Number
- no
- Prime Index
- 121
- Perfect Power
- no
- Smooth Number
- no
- Carmichael Number
- no
Classification
- Even
- no
- Odd
- yes
Names
- German
- sechshunderteinundsechzig· ordinal: sechshunderteinundsechzigste
- English
- six hundred sixty-one· ordinal: six hundred sixty-first
- Spanish
- seiscientos sesenta y uno· ordinal: 661º
- French
- six cent soixante et un· ordinal: six cent soixante et unième
- Italian
- seicentosessantuno· ordinal: 661º
- Latin
- sescenti sexaginta unus· ordinal: 661.
- Portuguese
- seiscentos e sessenta e um· ordinal: 661º
Appears in sequences
- Numbers k such that (1,k) is "good".at n=16A000696
- Primes p of the form 3k+1 such that Sum_{x=1..p} cos(2*Pi*x^3/p) > sqrt(p).at n=27A000921
- Primes with primitive root 2.at n=49A001122
- Numbers k such that k! - (k-1)! + (k-2)! - (k-3)! + ... - (-1)^k*1! is prime.at n=14A001272
- Related to graded partially ordered sets.at n=3A001830
- The coding-theoretic function A(n,4,3).at n=63A001839
- a(n) = least value of m for which Liouville's function A002819(m) = -n.at n=25A002053
- Denominators of continued fraction convergents to fifth root of 5.at n=8A002363
- Largest prime factor of n! + 1.at n=8A002583
- Centered 12-gonal numbers, or centered dodecagonal numbers: numbers of the form 6*k*(k-1) + 1.at n=10A003154
- Number of compositions of n such that no two adjacent parts are equal (these are sometimes called Carlitz compositions).at n=13A003242
- Numbers that are the sum of 6 positive 4th powers.at n=51A003340
- Discriminants of real quadratic fields with narrow class number 1.at n=53A003655
- Divisible only by primes congruent to 1 mod 5.at n=32A004615
- Divisible only by primes congruent to 3 mod 7.at n=41A004621
- Divisible only by primes congruent to 5 mod 8.at n=44A004627
- Class 4+ primes (for definition see A005105).at n=4A005108
- Class 3- primes (for definition see A005109).at n=33A005111
- Representation degeneracies for Neveu-Schwarz strings.at n=13A005296
- Primes p such that 2p-1 is also prime.at n=26A005382