21721519 has 2 divisors, whose sum is σ = 21721520. Its totient is φ = 21721518.

The previous prime is 21721517. The next prime is 21721549. The reversal of 21721519 is 91512712.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21721519 - 2¹ = 21721517 is a prime.

It is a super-2 number, since 2×21721519² = 943648775334722, which contains 22 as substring.

Together with 21721517, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21721517) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10860759 + 10860760.

It is an arithmetic number, because the mean of its divisors is an integer number (10860760).

Almost surely, 2^21721519 is an apocalyptic number.

21721519 is a deficient number, since it is larger than the sum of its proper divisors (1).

21721519 is an equidigital number, since it uses as much as digits as its factorization.

21721519 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 1260, while the sum is 28.

The square root of 21721519 is about 4660.6350425666. The cubic root of 21721519 is about 279.0166140764.

It can be divided in two parts, 217 and 21519, that added together give a triangular number (21736 = T₂₀₈).

The spelling of 21721519 in words is "twenty-one million, seven hundred twenty-one thousand, five hundred nineteen".