21451781 has 4 divisors (see below), whose sum is σ = 23101932. Its totient is φ = 19801632.

The previous prime is 21451769. The next prime is 21451789. The reversal of 21451781 is 18715412.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 11115556 + 10336225 = 3334^2 + 3215^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21451781 is a prime.

It is a super-2 number, since 2×21451781² = 920357816143922, 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 (21451789) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 825056 + ... + 825081.

It is an arithmetic number, because the mean of its divisors is an integer number (5775483).

Almost surely, 2^21451781 is an apocalyptic number.

It is an amenable number.

21451781 is a deficient number, since it is larger than the sum of its proper divisors (1650151).

21451781 is a wasteful number, since it uses less digits than its factorization.

21451781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1650150.

The product of its digits is 2240, while the sum is 29.

The square root of 21451781 is about 4631.6067406463. The cubic root of 21451781 is about 277.8568565906.

The spelling of 21451781 in words is "twenty-one million, four hundred fifty-one thousand, seven hundred eighty-one".