21051991 has 2 divisors, whose sum is σ = 21051992. Its totient is φ = 21051990.

The previous prime is 21051977. The next prime is 21052039. The reversal of 21051991 is 19915012.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21051991 - 2⁹ = 21051479 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21051091) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10525995 + 10525996.

It is an arithmetic number, because the mean of its divisors is an integer number (10525996).

Almost surely, 2^21051991 is an apocalyptic number.

21051991 is a deficient number, since it is larger than the sum of its proper divisors (1).

21051991 is an equidigital number, since it uses as much as digits as its factorization.

21051991 is an evil number, because the sum of its binary digits is even.

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

The square root of 21051991 is about 4588.2448714078. The cubic root of 21051991 is about 276.1199113137.

It can be divided in two parts, 2105 and 1991, that added together give a 12-th power (4096 = 2¹²).

The spelling of 21051991 in words is "twenty-one million, fifty-one thousand, nine hundred ninety-one".