21681291 has 16 divisors (see below), whose sum is σ = 29560608. Its totient is φ = 14131200.

The previous prime is 21681287. The next prime is 21681301. The reversal of 21681291 is 19218612.

It is a happy number.

It is not a de Polignac number, because 21681291 - 2² = 21681287 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21681211) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 46801 + ... + 47261.

It is an arithmetic number, because the mean of its divisors is an integer number (1847538).

Almost surely, 2^21681291 is an apocalyptic number.

21681291 is a deficient number, since it is larger than the sum of its proper divisors (7879317).

21681291 is a wasteful number, since it uses less digits than its factorization.

21681291 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 782.

The product of its digits is 1728, while the sum is 30.

The square root of 21681291 is about 4656.3173216610. The cubic root of 21681291 is about 278.8442624625.

The spelling of 21681291 in words is "twenty-one million, six hundred eighty-one thousand, two hundred ninety-one".