21391757 has 2 divisors, whose sum is σ = 21391758. Its totient is φ = 21391756.

The previous prime is 21391751. The next prime is 21391787. The reversal of 21391757 is 75719312.

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., 18412681 + 2979076 = 4291^2 + 1726^2 .

It is a cyclic number.

It is not a de Polignac number, because 21391757 - 2¹² = 21387661 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21391751) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10695878 + 10695879.

It is an arithmetic number, because the mean of its divisors is an integer number (10695879).

Almost surely, 2^21391757 is an apocalyptic number.

It is an amenable number.

21391757 is a deficient number, since it is larger than the sum of its proper divisors (1).

21391757 is an equidigital number, since it uses as much as digits as its factorization.

21391757 is an evil number, because the sum of its binary digits is even.

The product of its digits is 13230, while the sum is 35.

The square root of 21391757 is about 4625.1223767593. The cubic root of 21391757 is about 277.5974583962.

The spelling of 21391757 in words is "twenty-one million, three hundred ninety-one thousand, seven hundred fifty-seven".