20509 has 2 divisors, whose sum is σ = 20510. Its totient is φ = 20508.
The previous prime is 20507. The next prime is 20521. The reversal of 20509 is 90502.
It is a weak prime.
It can be written as a sum of positive squares in only one way, i.e., 14884 + 5625 = 122^2 + 75^2 .
It is a cyclic number.
It is not a de Polignac number, because 20509 - 21 = 20507 is a prime.
Together with 20507, it forms a pair of twin primes.
20509 is a lucky number.
It is a plaindrome in base 11.
It is a nialpdrome in base 12.
It is a junction number, because it is equal to n+sod(n) for n = 20492 and 20501.
It is a congruent number.
It is not a weakly prime, because it can be changed into another prime (20507) by changing a digit.
It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10254 + 10255.
It is an arithmetic number, because the mean of its divisors is an integer number (10255).
It is an amenable number.
20509 is a deficient number, since it is larger than the sum of its proper divisors (1).
20509 is an equidigital number, since it uses as much as digits as its factorization.
20509 is an evil number, because the sum of its binary digits is even.
The product of its (nonzero) digits is 90, while the sum is 16.
The square root of 20509 is about 143.2096365473. The cubic root of 20509 is about 27.3725229370.
It can be divided in two parts, 20 and 509, that added together give a square (529 = 232).
The spelling of 20509 in words is "twenty thousand, five hundred nine".
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