21555781 has 6 divisors (see below), whose sum is σ = 23341650. Its totient is φ = 19897488.

The previous prime is 21555739. The next prime is 21555797. The reversal of 21555781 is 18755512.

It can be written as a sum of positive squares in 3 ways, for example, as 2772225 + 18783556 = 1665^2 + 4334^2 .

It is not a de Polignac number, because 21555781 - 2²¹ = 19458629 is a prime.

It is a super-3 number, since 3×21555781³ (a number of 23 digits) contains 333 as substring. Note that it is a super-d number also for d = 2.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21555701) 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 5 ways as a sum of consecutive naturals, for example, 63606 + ... + 63943.

It is an arithmetic number, because the mean of its divisors is an integer number (3890275).

Almost surely, 2^21555781 is an apocalyptic number.

It is an amenable number.

21555781 is a deficient number, since it is larger than the sum of its proper divisors (1785869).

21555781 is a wasteful number, since it uses less digits than its factorization.

21555781 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 127575 (or 127562 counting only the distinct ones).

The product of its digits is 14000, while the sum is 34.

The square root of 21555781 is about 4642.8203712830. The cubic root of 21555781 is about 278.3051572069.

The spelling of 21555781 in words is "twenty-one million, five hundred fifty-five thousand, seven hundred eighty-one".