21042691 has 4 divisors (see below), whose sum is σ = 21074632. Its totient is φ = 21010752.

The previous prime is 21042689. The next prime is 21042731. The reversal of 21042691 is 19624012.

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

It is a cyclic number.

It is not a de Polignac number, because 21042691 - 2¹ = 21042689 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21042601) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 14961 + ... + 16306.

It is an arithmetic number, because the mean of its divisors is an integer number (5268658).

Almost surely, 2^21042691 is an apocalyptic number.

21042691 is a deficient number, since it is larger than the sum of its proper divisors (31941).

21042691 is an equidigital number, since it uses as much as digits as its factorization.

21042691 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 31940.

The product of its (nonzero) digits is 864, while the sum is 25.

The square root of 21042691 is about 4587.2313000327. The cubic root of 21042691 is about 276.0792454307.

It can be divided in two parts, 210426 and 91, that multiplied together give a triangular number (19148766 = T₆₁₈₈).

The spelling of 21042691 in words is "twenty-one million, forty-two thousand, six hundred ninety-one".