21401991 has 18 divisors (see below), whose sum is σ = 33477288. Its totient is φ = 13169520.

The previous prime is 21401987. The next prime is 21401993. The reversal of 21401991 is 19910412.

It is not a de Polignac number, because 21401991 - 2² = 21401987 is a prime.

It is a Smith number, since the sum of its digits (27) coincides with the sum of the digits of its prime factors.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21401993) 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 17 ways as a sum of consecutive naturals, for example, 5515 + ... + 8556.

Almost surely, 2^21401991 is an apocalyptic number.

21401991 is a deficient number, since it is larger than the sum of its proper divisors (12075297).

21401991 is a wasteful number, since it uses less digits than its factorization.

21401991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 14103 (or 14087 counting only the distinct ones).

The product of its (nonzero) digits is 648, while the sum is 27.

The square root of 21401991 is about 4626.2285935738. The cubic root of 21401991 is about 277.6417196736.

The spelling of 21401991 in words is "twenty-one million, four hundred one thousand, nine hundred ninety-one".