21505537309 has 2 divisors, whose sum is σ = 21505537310. Its totient is φ = 21505537308.

The previous prime is 21505537307. The next prime is 21505537367. The reversal of 21505537309 is 90373550512.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 19023305625 + 2482231684 = 137925^2 + 49822^2 .

It is a cyclic number.

It is not a de Polignac number, because 21505537309 - 2¹ = 21505537307 is a prime.

It is a super-3 number, since 3×21505537309³ (a number of 32 digits) contains 333 as substring. Note that it is a super-d number also for d = 2.

Together with 21505537307, 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 (21505537307) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10752768654 + 10752768655.

It is an arithmetic number, because the mean of its divisors is an integer number (10752768655).

Almost surely, 2^21505537309 is an apocalyptic number.

It is an amenable number.

21505537309 is a deficient number, since it is larger than the sum of its proper divisors (1).

21505537309 is an equidigital number, since it uses as much as digits as its factorization.

21505537309 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 141750, while the sum is 40.

The spelling of 21505537309 in words is "twenty-one billion, five hundred five million, five hundred thirty-seven thousand, three hundred nine".