21091951 has 4 divisors (see below), whose sum is σ = 22332672. Its totient is φ = 19851232.

The previous prime is 21091949. The next prime is 21091979. The reversal of 21091951 is 15919012.

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 21091951 - 2¹ = 21091949 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 (21091901) 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, 620335 + ... + 620368.

It is an arithmetic number, because the mean of its divisors is an integer number (5583168).

Almost surely, 2^21091951 is an apocalyptic number.

21091951 is a deficient number, since it is larger than the sum of its proper divisors (1240721).

21091951 is a wasteful number, since it uses less digits than its factorization.

21091951 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1240720.

The product of its (nonzero) digits is 810, while the sum is 28.

The square root of 21091951 is about 4592.5974132293. The cubic root of 21091951 is about 276.2945072738.

The spelling of 21091951 in words is "twenty-one million, ninety-one thousand, nine hundred fifty-one".