21099981 has 8 divisors (see below), whose sum is σ = 32152384. Its totient is φ = 12057120.

The previous prime is 21099977. The next prime is 21100003. The reversal of 21099981 is 18999012.

It is a happy number.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21099981 - 2² = 21099977 is a prime.

It is a super-2 number, since 2×21099981² = 890418396400722, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21091981) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 502360 + ... + 502401.

It is an arithmetic number, because the mean of its divisors is an integer number (4019048).

Almost surely, 2^21099981 is an apocalyptic number.

21099981 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21099981 is a deficient number, since it is larger than the sum of its proper divisors (11052403).

21099981 is a wasteful number, since it uses less digits than its factorization.

21099981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1004771.

The product of its (nonzero) digits is 11664, while the sum is 39.

The square root of 21099981 is about 4593.4715629902. The cubic root of 21099981 is about 276.3295658829.

It can be divided in two parts, 2109 and 9981, that added together give a triangular number (12090 = T₁₅₅).

The spelling of 21099981 in words is "twenty-one million, ninety-nine thousand, nine hundred eighty-one".