21505766501 has 4 divisors (see below), whose sum is σ = 21727475532. Its totient is φ = 21284057472.

The previous prime is 21505766467. The next prime is 21505766519. The reversal of 21505766501 is 10566750512.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 3562896100 + 17942870401 = 59690^2 + 133951^2 .

It is a cyclic number.

It is not a de Polignac number, because 21505766501 - 2¹⁴ = 21505750117 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21505766551) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 110854370 + ... + 110854563.

It is an arithmetic number, because the mean of its divisors is an integer number (5431868883).

Almost surely, 2^21505766501 is an apocalyptic number.

It is an amenable number.

21505766501 is a deficient number, since it is larger than the sum of its proper divisors (221709031).

21505766501 is an equidigital number, since it uses as much as digits as its factorization.

21505766501 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 221709030.

The product of its (nonzero) digits is 63000, while the sum is 38.

The spelling of 21505766501 in words is "twenty-one billion, five hundred five million, seven hundred sixty-six thousand, five hundred one".