21505766551 has 2 divisors, whose sum is σ = 21505766552. Its totient is φ = 21505766550.

The previous prime is 21505766549. The next prime is 21505766563. The reversal of 21505766551 is 15566750512.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21505766551 - 2¹ = 21505766549 is a prime.

It is a super-2 number, since 2×21505766551² (a number of 21 digits) contains 22 as substring.

Together with 21505766549, it forms a pair of twin primes.

It is a junction number, because it is equal to n+sod(n) for n = 21505766498 and 21505766507.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21505766651) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10752883275 + 10752883276.

It is an arithmetic number, because the mean of its divisors is an integer number (10752883276).

Almost surely, 2^21505766551 is an apocalyptic number.

21505766551 is a deficient number, since it is larger than the sum of its proper divisors (1).

21505766551 is an equidigital number, since it uses as much as digits as its factorization.

21505766551 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 315000, while the sum is 43.

The spelling of 21505766551 in words is "twenty-one billion, five hundred five million, seven hundred sixty-six thousand, five hundred fifty-one".