21970 has 16 divisors (see below), whose sum is σ = 42840. Its totient is φ = 8112.

The previous prime is 21961. The next prime is 21977. The reversal of 21970 is 7912.

It can be written as a sum of positive squares in 4 ways, for example, as 361 + 21609 = 19^2 + 147^2 .

It is a Smith number, since the sum of its digits (19) coincides with the sum of the digits of its prime factors.

It is an Ulam number.

It is a plaindrome in base 15.

It is a nialpdrome in base 13.

It is not an unprimeable number, because it can be changed into a prime (21977) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 1684 + ... + 1696.

2²¹⁹⁷⁰ is an apocalyptic number.

21970 is a deficient number, since it is larger than the sum of its proper divisors (20870).

21970 is an equidigital number, since it uses as much as digits as its factorization.

21970 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 46 (or 20 counting only the distinct ones).

The product of its (nonzero) digits is 126, while the sum is 19.

The square root of 21970 is about 148.2228052629. Note that the first 3 decimals coincide. The cubic root of 21970 is about 28.0076509704.

It can be divided in two parts, 219 and 70, that added together give a square (289 = 17²).

The spelling of 21970 in words is "twenty-one thousand, nine hundred seventy".