21001531 has 2 divisors, whose sum is σ = 21001532. Its totient is φ = 21001530.

The previous prime is 21001529. The next prime is 21001549. The reversal of 21001531 is 13510012.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21001531 - 2¹ = 21001529 is a prime.

It is a super-2 number, since 2×21001531² = 882128608687922, which contains 22 as substring.

Together with 21001529, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (21001831) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10500765 + 10500766.

It is an arithmetic number, because the mean of its divisors is an integer number (10500766).

Almost surely, 2^21001531 is an apocalyptic number.

21001531 is a deficient number, since it is larger than the sum of its proper divisors (1).

21001531 is an equidigital number, since it uses as much as digits as its factorization.

21001531 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 30, while the sum is 13.

The square root of 21001531 is about 4582.7427377063. The cubic root of 21001531 is about 275.8991220989.

Adding to 21001531 its reverse (13510012), we get a palindrome (34511543).

The spelling of 21001531 in words is "twenty-one million, one thousand, five hundred thirty-one".