21221991 has 12 divisors (see below), whose sum is σ = 35033232. Its totient is φ = 12126816.

The previous prime is 21221953. The next prime is 21222013. The reversal of 21221991 is 19912212.

It is not a de Polignac number, because 21221991 - 2⁶ = 21221927 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21221791) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 168366 + ... + 168491.

It is an arithmetic number, because the mean of its divisors is an integer number (2919436).

Almost surely, 2^21221991 is an apocalyptic number.

21221991 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

21221991 is a deficient number, since it is larger than the sum of its proper divisors (13811241).

21221991 is a wasteful number, since it uses less digits than its factorization.

21221991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 336870 (or 336867 counting only the distinct ones).

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

The square root of 21221991 is about 4606.7332243142. The cubic root of 21221991 is about 276.8611649877.

The spelling of 21221991 in words is "twenty-one million, two hundred twenty-one thousand, nine hundred ninety-one".