21375991 has 16 divisors (see below), whose sum is σ = 26417664. Its totient is φ = 16843104.

The previous prime is 21375971. The next prime is 21376009. The reversal of 21375991 is 19957312.

21375991 is a nontrivial binomial coefficient, being equal to C(6539, 2).

It is a cyclic number.

It is not a de Polignac number, because 21375991 - 2²¹ = 19278839 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21375991.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21375961) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 42246 + ... + 42748.

It is an arithmetic number, because the mean of its divisors is an integer number (1651104).

Almost surely, 2^21375991 is an apocalyptic number.

21375991 is the 6538-th triangular number.

21375991 is the 2180-th centered nonagonal number.

21375991 is a deficient number, since it is larger than the sum of its proper divisors (5041673).

21375991 is a wasteful number, since it uses less digits than its factorization.

21375991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 990.

The product of its digits is 17010, while the sum is 37.

The square root of 21375991 is about 4623.4176752701. The cubic root of 21375991 is about 277.5292440030.

It can be divided in two parts, 21375 and 991, that added together give a triangular number (22366 = T₂₁₁).

The spelling of 21375991 in words is "twenty-one million, three hundred seventy-five thousand, nine hundred ninety-one".