20991961 has 2 divisors, whose sum is σ = 20991962. Its totient is φ = 20991960.

The previous prime is 20991947. The next prime is 20991967. The reversal of 20991961 is 16919902.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20985561 + 6400 = 4581^2 + 80^2 .

It is a cyclic number.

It is not a de Polignac number, because 20991961 - 2¹³ = 20983769 is a prime.

It is a Chen prime.

It is a pancake number, because a pancake can be divided into 20991961 parts by 6479 straight cuts.

It is a self number, because there is not a number n which added to its sum of digits gives 20991961.

It is not a weakly prime, because it can be changed into another prime (20991967) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10495980 + 10495981.

It is an arithmetic number, because the mean of its divisors is an integer number (10495981).

Almost surely, 2^20991961 is an apocalyptic number.

It is an amenable number.

20991961 is a deficient number, since it is larger than the sum of its proper divisors (1).

20991961 is an equidigital number, since it uses as much as digits as its factorization.

20991961 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 8748, while the sum is 37.

The square root of 20991961 is about 4581.6984841868. The cubic root of 20991961 is about 275.8572083966.

It can be divided in two parts, 20991 and 961, that added together give a cube (21952 = 28³).

The spelling of 20991961 in words is "twenty million, nine hundred ninety-one thousand, nine hundred sixty-one".