1991
domain: N
Properties
Digital Properties
- Digit Count
- 4
- Digit Sum
- 20
- Digital Root
- 2
- Palindromic Number
- yes
- Repdigit
- no
- Automorphic
- no
- Kaprekar Number
- no
- Multiplicative Persistence
- 2
Divisibility
- Divisor Count
- 4
- Divisor Sum
- 2184
- Proper Divisor Sum (Aliquot Sum)
- 193
- 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
- 1800
- Möbius Function
- 1
- Radical
- 1991
- Omega Function (Ω)
- 2
- Little Omega Function (ω)
- 2
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
- 50
- Smith Number
- no
- Vampire Number
- no
Primality
- Prime
- no
- Composite Number
- yes
- Semiprime
- yes
- Squarefree Number
- yes
- Prime Power
- no
- Prime Factorization
- no
- Twin Prime
- no
- Mersenne Prime
- no
- Sophie Germain Prime
- no
- Safe Prime
- no
- Powerful Number
- no
- Achilles Number
- no
- Perfect Power
- no
- Smooth Number
- no
- Carmichael Number
- no
Classification
- Even
- no
- Odd
- yes
Appears in sequences
- Number of partitions of n in which no parts are multiples of 3.at n=33A000726
- a(n) = n*(3*n^2 - 1)/2.at n=11A004188
- a(n) = ceiling(n*phi^11), where phi is the golden ratio, A001622.at n=10A004966
- Engel expansion of Pi.at n=8A006784
- Coefficients in expansion of Pi as Sum_{n>=1} a(n)/(n*n!*(n+1)!), as found by greedy algorithm.at n=44A011191
- Bisection of A001400.at n=30A014125
- Positive integers n such that 2^n == 2^11 (mod n).at n=37A015935
- Pseudoprimes to base 42.at n=14A020170
- Pseudoprimes to base 46.at n=28A020174
- Pseudoprimes to base 56.at n=23A020184
- Pseudoprimes to base 59.at n=17A020187
- Strong pseudoprimes to base 42.at n=6A020268
- Strong pseudoprimes to base 46.at n=8A020272
- Strong pseudoprimes to base 59.at n=6A020285
- Ordered sequence of distinct terms of the form floor(x^i * floor(x^j)), where x = sqrt(2).at n=62A022768
- Numbers k such that Fibonacci(k) == 89 (mod k).at n=28A023182
- a(n) = Sum_{k=floor((n+1)/2)..n} T(k,n-k); i.e., a(n) is n-th diagonal sum of left-justified array T given by A027011.at n=17A027022
- Palindromic in bases 10 and 16.at n=15A029731
- Numbers whose base-10 representation has 2 fewer 0's than 9's.at n=41A031500
- Numbers k such that 41*2^k+1 is prime.at n=6A032370