21531709 has 2 divisors, whose sum is σ = 21531710. Its totient is φ = 21531708.

The previous prime is 21531689. The next prime is 21531721. The reversal of 21531709 is 90713512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 11628100 + 9903609 = 3410^2 + 3147^2 .

It is a cyclic number.

It is not a de Polignac number, because 21531709 - 2²¹ = 19434557 is a prime.

It is a super-2 number, since 2×21531709² = 927228984921362, which contains 22 as substring.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21531739) 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, 10765854 + 10765855.

It is an arithmetic number, because the mean of its divisors is an integer number (10765855).

Almost surely, 2^21531709 is an apocalyptic number.

It is an amenable number.

21531709 is a deficient number, since it is larger than the sum of its proper divisors (1).

21531709 is an equidigital number, since it uses as much as digits as its factorization.

21531709 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1890, while the sum is 28.

The square root of 21531709 is about 4640.2272573657. The cubic root of 21531709 is about 278.2015213256.

The spelling of 21531709 in words is "twenty-one million, five hundred thirty-one thousand, seven hundred nine".