21689531 has 2 divisors, whose sum is σ = 21689532. Its totient is φ = 21689530.

The previous prime is 21689527. The next prime is 21689543. The reversal of 21689531 is 13598612.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21689531 - 2² = 21689527 is a prime.

It is a super-4 number, since 4×21689531⁴ (a number of 30 digits) contains 4444 as substring. Note that it is a super-d number also for d = 2.

It is a junction number, because it is equal to n+sod(n) for n = 21689491 and 21689500.

It is not a weakly prime, because it can be changed into another prime (21689581) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10844765 + 10844766.

It is an arithmetic number, because the mean of its divisors is an integer number (10844766).

Almost surely, 2^21689531 is an apocalyptic number.

21689531 is a deficient number, since it is larger than the sum of its proper divisors (1).

21689531 is an equidigital number, since it uses as much as digits as its factorization.

21689531 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 12960, while the sum is 35.

The square root of 21689531 is about 4657.2020570295. The cubic root of 21689531 is about 278.8795830177.

The spelling of 21689531 in words is "twenty-one million, six hundred eighty-nine thousand, five hundred thirty-one".