261
domain: N
Properties
Digital Properties
- Digit Count
- 3
- Digit Sum
- 9
- Digital Root
- 9
- Palindromic Number
- no
- Repdigit
- no
- Automorphic
- no
- Kaprekar Number
- no
- Multiplicative Persistence
- 2
Divisibility
- Divisor Count
- 6
- Divisor Sum
- 390
- Proper Divisor Sum (Aliquot Sum)
- 129
- 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
- 168
- Möbius Function
- 0
- Radical
- 87
- Omega Function (Ω)
- 3
- 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
- yes
- Narcissistic Number
- no
- Collatz Steps
- 29
- Smith Number
- no
Primality
- Prime
- no
- Composite Number
- yes
- Semiprime
- no
- Squarefree Number
- no
- 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
- zweihunderteinundsechzig· ordinal: zweihunderteinundsechzigste
- English
- two hundred sixty-one· ordinal: two hundred sixty-first
- Spanish
- doscientos sesenta y uno· ordinal: 261º
- French
- deux cent soixante et un· ordinal: deux cent soixante et unième
- Italian
- duecentosessantuno· ordinal: 261º
- Latin
- ducenti sexaginta unus· ordinal: 261.
- Portuguese
- duzentos e sessenta e um· ordinal: 261º
Appears in sequences
- a(n) = floor(n^2/3).at n=28A000212
- Number of series-reduced planted trees with n leaves. Also the number of essentially series series-parallel networks with n edges; also the number of essentially parallel series-parallel networks with n edges.at n=7A000669
- Moser-de Bruijn sequence: sums of distinct powers of 4.at n=19A000695
- Lucky numbers.at n=49A000959
- Generalized octagonal numbers: k*(3*k-2), k=0, +- 1, +- 2, +-3, ...at n=18A001082
- Moran numbers: k such that k/(sum of digits of k) is prime.at n=24A001101
- 9-gonal (or enneagonal or nonagonal) numbers: a(n) = n*(7*n-5)/2.at n=9A001106
- Number of ways of making change for n cents using coins of 1, 2, 5, 10, 25 cents.at n=43A001301
- Number of ways of making change for n cents using coins of 1, 2, 5, 10, 25, 50 cents.at n=43A001302
- a(n) = Sum_{k=0..n} (k+1)! binomial(n,k).at n=4A001339
- a(n) = Sum_{k = 0..4} (n+k)! C(4,k).at n=2A001346
- a(n) is the number of partitions of n into at most 3 parts; also partitions of n+3 in which the greatest part is 3; also number of unlabeled multigraphs with 3 nodes and n edges.at n=53A001399
- Partial sums of A001462; also a(n) is the last occurrence of n in A001462.at n=36A001463
- Number of series-reduced rooted trees with n nodes.at n=13A001679
- Sorting numbers: maximal number of comparisons for sorting n elements by binary insertion.at n=53A001855
- Triangular numbers plus quarter-squares: n*(n+1)/2 + floor((n+1)^2/4) (i.e., A000217(n) + A002620(n+1)).at n=18A001859
- A Beatty sequence: floor(n * (sqrt(5) + 3)).at n=49A001962
- Numbers x such that x^2 + y^2 = p^2 = A002144(n)^2, x < y.at n=43A002366
- Numbers k such that (k^2 + k + 1)/7 is prime.at n=29A002641
- Numbers k such that (k^2 + 1)/2 is prime.at n=42A002731