21961 has 2 divisors, whose sum is σ = 21962. Its totient is φ = 21960.

The previous prime is 21943. The next prime is 21977. The reversal of 21961 is 16912.

It is the 61-st star number.

21961 is an esthetic number in base 7, because in such base its adjacent digits differ by 1.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20736 + 1225 = 144^2 + 35^2 .

It is a cyclic number.

It is not a de Polignac number, because 21961 - 2⁵ = 21929 is a prime.

It is a Chen prime.

It is a plaindrome in base 11.

It is a nialpdrome in base 9.

It is a zygodrome in base 9.

It is not a weakly prime, because it can be changed into another prime (21911) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10980 + 10981.

It is an arithmetic number, because the mean of its divisors is an integer number (10981).

2²¹⁹⁶¹ is an apocalyptic number.

It is an amenable number.

21961 is a deficient number, since it is larger than the sum of its proper divisors (1).

21961 is an equidigital number, since it uses as much as digits as its factorization.

21961 is an evil number, because the sum of its binary digits is even.

The product of its digits is 108, while the sum is 19.

The square root of 21961 is about 148.1924424524. The cubic root of 21961 is about 28.0038260078.

It can be divided in two parts, 2196 and 1, that added together give a cube (2197 = 13³).

The spelling of 21961 in words is "twenty-one thousand, nine hundred sixty-one".