21315997 has 2 divisors, whose sum is σ = 21315998. Its totient is φ = 21315996.

The previous prime is 21315989. The next prime is 21316021. The reversal of 21315997 is 79951312.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 12489156 + 8826841 = 3534^2 + 2971^2 .

It is a cyclic number.

It is not a de Polignac number, because 21315997 - 2³ = 21315989 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21315947) 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, 10657998 + 10657999.

It is an arithmetic number, because the mean of its divisors is an integer number (10657999).

Almost surely, 2^21315997 is an apocalyptic number.

It is an amenable number.

21315997 is a deficient number, since it is larger than the sum of its proper divisors (1).

21315997 is an equidigital number, since it uses as much as digits as its factorization.

21315997 is an odious number, because the sum of its binary digits is odd.

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

The square root of 21315997 is about 4616.9250589543. The cubic root of 21315997 is about 277.2693622420.

It can be divided in two parts, 2131 and 5997, that added together give a triangular number (8128 = T₁₂₇).

The spelling of 21315997 in words is "twenty-one million, three hundred fifteen thousand, nine hundred ninety-seven".