21505109 has 2 divisors, whose sum is σ = 21505110. Its totient is φ = 21505108.

The previous prime is 21505063. The next prime is 21505129. The reversal of 21505109 is 90150512.

21505109 is nontrivially palindromic in base 7.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21252100 + 253009 = 4610^2 + 503^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21505109 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21505129) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10752554 + 10752555.

It is an arithmetic number, because the mean of its divisors is an integer number (10752555).

Almost surely, 2^21505109 is an apocalyptic number.

It is an amenable number.

21505109 is a deficient number, since it is larger than the sum of its proper divisors (1).

21505109 is an equidigital number, since it uses as much as digits as its factorization.

21505109 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 450, while the sum is 23.

The square root of 21505109 is about 4637.3601326617. The cubic root of 21505109 is about 278.0869119075.

The spelling of 21505109 in words is "twenty-one million, five hundred five thousand, one hundred nine".