21051721 has 2 divisors, whose sum is σ = 21051722. Its totient is φ = 21051720.

The previous prime is 21051707. The next prime is 21051727. The reversal of 21051721 is 12715012.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17131321 + 3920400 = 4139^2 + 1980^2 .

It is a cyclic number.

It is not a de Polignac number, because 21051721 - 2¹⁷ = 20920649 is a prime.

It is equal to p_1333045 and since 21051721 and 1333045 have the same sum of digits, it is a Honaker prime.

It is not a weakly prime, because it can be changed into another prime (21051727) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10525860 + 10525861.

It is an arithmetic number, because the mean of its divisors is an integer number (10525861).

Almost surely, 2^21051721 is an apocalyptic number.

It is an amenable number.

21051721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21051721 is an equidigital number, since it uses as much as digits as its factorization.

21051721 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 140, while the sum is 19.

The square root of 21051721 is about 4588.2154482980. The cubic root of 21051721 is about 276.1187308601.

Adding to 21051721 its reverse (12715012), we get a palindrome (33766733).

The spelling of 21051721 in words is "twenty-one million, fifty-one thousand, seven hundred twenty-one".