1821
domain: N
Properties
Digital Properties
- Digit Count
- 4
- Digit Sum
- 12
- Digital Root
- 3
- Palindromic Number
- no
- Repdigit
- no
- Automorphic
- no
- Kaprekar Number
- no
- Multiplicative Persistence
- 2
Divisibility
- Divisor Count
- 4
- Divisor Sum
- 2432
- Proper Divisor Sum (Aliquot Sum)
- 611
- 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
- 1212
- Möbius Function
- 1
- Radical
- 1821
- 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
- yes
- Harshad Number
- no
- Narcissistic Number
- no
- Collatz Steps
- 42
- 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 asymmetrical planar partitions of n: planar partitions (A000219) that when regarded as 3-D objects have no symmetry.at n=15A000785
- Number of permutations of length n with longest increasing subsequence of length 4.at n=3A001455
- Mian-Chowla sequence (a B_2 sequence): a(1) = 1; for n>1, a(n) = smallest number > a(n-1) such that the pairwise sums of elements are all distinct.at n=33A005282
- Positions of remoteness 2 in Beans-Don't-Talk.at n=5A005698
- Coordination sequence T2 for Zeolite Code AFO.at n=28A008016
- Coordination sequence T7 for Zeolite Code MFI.at n=27A008170
- Numbers k such that the continued fraction for sqrt(k) has period 54.at n=2A020393
- a(n) = least m such that if r and s in {1/1, 1/4, 1/7, ..., 1/(3n-2)} satisfy r < s, then r < k/m < (k+1)/m < s for some integer k.at n=19A024836
- Number of partitions of n with distinct parts p(i) such that if i != j, then |p(i) - p(j)| >= 3.at n=65A025157
- [ Sum{(sqrt(j+1)-sqrt(i+1))^3} ], 1 <= i < j <= n.at n=24A025223
- Numbers that are the sum of 3 nonzero squares in exactly 9 ways.at n=42A025329
- Numbers that are the sum of 3 distinct nonzero squares in exactly 9 ways.at n=27A025347
- a(n) = dot_product(1,2,...,n)*(3,4,...,n,1,2).at n=15A026037
- a(n) = Sum_{k=n..2*n} T(n,k), T given by A027052.at n=8A027067
- Number of subgroups of index n of fundamental group of the non-orientable cycle bundle over the Klein bottle.at n=51A027844
- Graham-Sloane-type lower bound on the size of a ternary (n,3,3) constant-weight code.at n=51A030503
- Numbers k such that the continued fraction for sqrt(k) has even period and if the last term of the periodic part is deleted the central term is 28.at n=14A031526
- Concatenation of n and n + 3.at n=17A032608
- Decimal part of a(n)^(1/3) starts with reversal of its integer part: first term of runs.at n=10A034309
- Zeckendorf expansion of n: repeatedly subtract the largest Fibonacci number you can until nothing remains. Little-endian concatenation of decimals.at n=30A035515