Properties of the Number 21991

Twenty-One Thousand Nine Hundred Ninety-One

Basics

Value: 21990 → 21991 → 21992

Parity: odd

Prime: Yes

Previous Prime: 21977

Next Prime: 21997

Digit Sum: 22

Digital Root: 4

Palindrome: No

Factorization: 21991

Divisors: 1, 21991

Polygonal

Triangular: No

Square: No

Pentagonal: No

Hexagonal: No

Heptagonal: No

Tetrahedral: No

Today's Target

2196 Try Today's Challenge

Representations

Binary: 101010111100111

Octal: 52747

Duodecimal: 10887

Hexadecimal: 55e7

Square: 483604081

Square Root: 148.29362764461592

Classification

Fibonacci: No

Bell Number: No

Factorial Number: No

Regular Number: No

Perfect Number: No

Special

Kaprekar Constant: No

Munchausen Constant: No

Armstrong Number: No

Kaprekar Number: No

Catalan Number: No

Vampire Number: No

Taxicab Number: No

Super Prime: No

Friedman Number: No

Fermat Number: No

Cullen Number: No

OEIS Sequences

Primes having only {1, 2, 9} as digits. A385776

T(n,k)=Number of (n+1)X(k+1) 0..2 arrays x(i,j) with row sums sum{j·x(i,j), j=1..k+1} nondecreasing, and column sums sum{i·x(i,j), i=1..n+1} nondecreasing. A232778

Index where n first appears in A381597. A381599

a(n) is the largest prime p such that b(n) = b(n-1)*(p+1)/(p-1) is an integer (A385959), where b(0) = 1. A385958

Primes that remain prime through 3 iterations of function f(x) = 2x + 9. A23276

Primes that remain prime through 4 iterations of function f(x) = 2x + 9. A23306

Primes that remain prime through 5 iterations of function f(x) = 2x + 9. A23334

Primes such that applying "reverse and add" twice produces two more primes. A174402

Number of nondecreasing arrangements of 4 numbers in -(n+2)..(n+2) with sum zero. A188212

Number of nX7 0..1 arrays avoiding 0 0 0 and 0 1 1 horizontally and 0 1 1 and 1 1 0 vertically. A207487