21841 has 2 divisors, whose sum is σ = 21842. Its totient is φ = 21840.

The previous prime is 21839. The next prime is 21851. The reversal of 21841 is 14812.

21841 = 104² + 105².

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 11025 + 10816 = 105^2 + 104^2 .

It is a cyclic number.

It is not a de Polignac number, because 21841 - 2¹ = 21839 is a prime.

Together with 21839, it forms a pair of twin primes.

21841 is a lucky number.

It is a nialpdrome in base 16.

It is not a weakly prime, because it can be changed into another prime (21821) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10920 + 10921.

It is an arithmetic number, because the mean of its divisors is an integer number (10921).

2²¹⁸⁴¹ is an apocalyptic number.

21841 is the 105-th centered square number.

It is an amenable number.

21841 is a deficient number, since it is larger than the sum of its proper divisors (1).

21841 is an equidigital number, since it uses as much as digits as its factorization.

21841 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 64, while the sum is 16.

The square root of 21841 is about 147.7870089013. The cubic root of 21841 is about 27.9527263532.

The spelling of 21841 in words is "twenty-one thousand, eight hundred forty-one".