21668261 has 2 divisors, whose sum is σ = 21668262. Its totient is φ = 21668260.

The previous prime is 21668221. The next prime is 21668263. The reversal of 21668261 is 16286612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21436900 + 231361 = 4630^2 + 481^2 .

It is a cyclic number.

It is not a de Polignac number, because 21668261 - 2⁶ = 21668197 is a prime.

Together with 21668263, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21668263) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10834130 + 10834131.

It is an arithmetic number, because the mean of its divisors is an integer number (10834131).

Almost surely, 2^21668261 is an apocalyptic number.

It is an amenable number.

21668261 is a deficient number, since it is larger than the sum of its proper divisors (1).

21668261 is an equidigital number, since it uses as much as digits as its factorization.

21668261 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 6912, while the sum is 32.

The square root of 21668261 is about 4654.9179369781. The cubic root of 21668261 is about 278.7883914224.

The spelling of 21668261 in words is "twenty-one million, six hundred sixty-eight thousand, two hundred sixty-one".