21721057 has 2 divisors, whose sum is σ = 21721058. Its totient is φ = 21721056.

The previous prime is 21721031. The next prime is 21721067. The reversal of 21721057 is 75012712.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19018321 + 2702736 = 4361^2 + 1644^2 .

It is a cyclic number.

It is not a de Polignac number, because 21721057 - 2⁷ = 21720929 is a prime.

It is not a weakly prime, because it can be changed into another prime (21721067) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10860528 + 10860529.

It is an arithmetic number, because the mean of its divisors is an integer number (10860529).

Almost surely, 2^21721057 is an apocalyptic number.

It is an amenable number.

21721057 is a deficient number, since it is larger than the sum of its proper divisors (1).

21721057 is an equidigital number, since it uses as much as digits as its factorization.

21721057 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 980, while the sum is 25.

The square root of 21721057 is about 4660.5854782420. The cubic root of 21721057 is about 279.0146359061.

Adding to 21721057 its reverse (75012712), we get a palindrome (96733769).

The spelling of 21721057 in words is "twenty-one million, seven hundred twenty-one thousand, fifty-seven".