781
domain: N
Properties
Digital Properties
- Digit Count
- 3
- Digit Sum
- 16
- Digital Root
- 7
- Palindromic Number
- no
- Repdigit
- no
- Automorphic
- no
- Kaprekar Number
- no
- Multiplicative Persistence
- 3
Divisibility
- Divisor Count
- 4
- Divisor Sum
- 864
- Proper Divisor Sum (Aliquot Sum)
- 83
- 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
- 700
- Möbius Function
- 1
- Radical
- 781
- 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
- 121
- Smith 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
Names
- German
- siebenhunderteinundachtzig· ordinal: siebenhunderteinundachtzigste
- English
- seven hundred eighty-one· ordinal: seven hundred eighty-first
- Spanish
- setecientos ochenta y uno· ordinal: 781º
- French
- sept cent quatre-vingt-un· ordinal: sept cent quatre-vingt-unième
- Italian
- settecentoottantuno· ordinal: 781º
- Latin
- septingenti octoginta unus· ordinal: 781.
- Portuguese
- setecentos e oitenta e um· ordinal: 781º
Appears in sequences
- Central polygonal numbers (the Lazy Caterer's sequence): n(n+1)/2 + 1; or, maximal number of pieces formed when slicing a pancake with n cuts.at n=39A000124
- a(n) = (p^p-1)/(p-1) where p = prime(n).at n=2A001039
- Odd-indexed terms of A124296.at n=3A001603
- Numbers k such that 9*2^k - 1 is prime.at n=16A002236
- Numerators of convergents to cube root of 4.at n=8A002356
- a(n) = (5^n - 1)/4.at n=5A003463
- a(n) = floor((n^2 + 6n - 3)/4).at n=52A004116
- Number of partitions of n into 3 or more parts.at n=20A004250
- Divisible only by primes congruent to 1 mod 5.at n=38A004615
- a(n) = 4^n - 3^n.at n=5A005061
- Number of domino tilings of 4 X (n-1) board.at n=8A005178
- Denominators of continued fraction convergents to cube root of 7.at n=5A005485
- Pseudoprimes to base 5.at n=4A005936
- Gaussian binomial coefficient [ n,4 ] for q = 5.at n=1A006113
- a(n) = 6*a(n-2) - a(n-4).at n=9A006452
- 4-dimensional analog of centered polygonal numbers. Also number of regions created by sides and diagonals of a convex n-gon in general position.at n=13A006522
- 5th-order maximal independent sets in cycle graph.at n=37A007388
- Number of regions in regular n-gon with all diagonals drawn.at n=12A007678
- Coordination sequence T2 for Zeolite Code AFY.at n=23A008030
- Coordination sequence T2 for Coesite.at n=15A008268