21001543 has 4 divisors (see below), whose sum is σ = 21044808. Its totient is φ = 20958280.

The previous prime is 21001531. The next prime is 21001549. The reversal of 21001543 is 34510012.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21001543 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21001549) 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 in 3 ways as a sum of consecutive naturals, for example, 20896 + ... + 21877.

It is an arithmetic number, because the mean of its divisors is an integer number (5261202).

Almost surely, 2^21001543 is an apocalyptic number.

21001543 is a deficient number, since it is larger than the sum of its proper divisors (43265).

21001543 is an equidigital number, since it uses as much as digits as its factorization.

21001543 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 43264.

The product of its (nonzero) digits is 120, while the sum is 16.

The square root of 21001543 is about 4582.7440469657. The cubic root of 21001543 is about 275.8991746472.

Adding to 21001543 its reverse (34510012), we get a palindrome (55511555).

The spelling of 21001543 in words is "twenty-one million, one thousand, five hundred forty-three".