21505 has 16 divisors (see below), whose sum is σ = 31104. Its totient is φ = 14080.

The previous prime is 21503. The next prime is 21517. The reversal of 21505 is 50512.

It is not a de Polignac number, because 21505 - 2¹ = 21503 is a prime.

It is a Duffinian number.

21505 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (21503) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 924 + ... + 946.

It is an arithmetic number, because the mean of its divisors is an integer number (1944).

It is a Proth number, since it is equal to 21 ⋅ 2¹⁰ + 1 and 21 < 2¹⁰.

2²¹⁵⁰⁵ is an apocalyptic number.

21505 is the 85-th octagonal number.

It is an amenable number.

21505 is a deficient number, since it is larger than the sum of its proper divisors (9599).

21505 is a wasteful number, since it uses less digits than its factorization.

21505 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 56.

The product of its (nonzero) digits is 50, while the sum is 13.

The square root of 21505 is about 146.6458318535. The cubic root of 21505 is about 27.8086442073.

The spelling of 21505 in words is "twenty-one thousand, five hundred five".