21515381 has 2 divisors, whose sum is σ = 21515382. Its totient is φ = 21515380.

The previous prime is 21515369. The next prime is 21515411. The reversal of 21515381 is 18351512.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21298225 + 217156 = 4615^2 + 466^2 .

It is a cyclic number.

It is not a de Polignac number, because 21515381 - 2⁶ = 21515317 is a prime.

It is a super-2 number, since 2×21515381² = 925823239150322, which contains 22 as substring.

It is a Chen prime.

It is equal to p_1360493 and since 21515381 and 1360493 have the same sum of digits, it is a Honaker prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21515381.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21515321) 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 as a sum of consecutive naturals, namely, 10757690 + 10757691.

It is an arithmetic number, because the mean of its divisors is an integer number (10757691).

Almost surely, 2^21515381 is an apocalyptic number.

It is an amenable number.

21515381 is a deficient number, since it is larger than the sum of its proper divisors (1).

21515381 is an equidigital number, since it uses as much as digits as its factorization.

21515381 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 1200, while the sum is 26.

The square root of 21515381 is about 4638.4675271042. The cubic root of 21515381 is about 278.1311812959.

Adding to 21515381 its reverse (18351512), we get a palindrome (39866893).

The spelling of 21515381 in words is "twenty-one million, five hundred fifteen thousand, three hundred eighty-one".