21255557 has 2 divisors, whose sum is σ = 21255558. Its totient is φ = 21255556.

The previous prime is 21255541. The next prime is 21255569. The reversal of 21255557 is 75555212.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 16924996 + 4330561 = 4114^2 + 2081^2 .

It is a cyclic number.

It is not a de Polignac number, because 21255557 - 2⁴ = 21255541 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21255587) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10627778 + 10627779.

It is an arithmetic number, because the mean of its divisors is an integer number (10627779).

Almost surely, 2^21255557 is an apocalyptic number.

It is an amenable number.

21255557 is a deficient number, since it is larger than the sum of its proper divisors (1).

21255557 is an equidigital number, since it uses as much as digits as its factorization.

21255557 is an evil number, because the sum of its binary digits is even.

The product of its digits is 17500, while the sum is 32.

The square root of 21255557 is about 4610.3749305235. The cubic root of 21255557 is about 277.0070549537.

It can be divided in two parts, 21255 and 557, that added together give a palindrome (21812).

The spelling of 21255557 in words is "twenty-one million, two hundred fifty-five thousand, five hundred fifty-seven".