21020420261 has 2 divisors, whose sum is σ = 21020420262. Its totient is φ = 21020420260.

The previous prime is 21020420249. The next prime is 21020420293. The reversal of 21020420261 is 16202402012.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 19703456161 + 1316964100 = 140369^2 + 36290^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21020420261 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21020420221) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10510210130 + 10510210131.

It is an arithmetic number, because the mean of its divisors is an integer number (10510210131).

Almost surely, 2^21020420261 is an apocalyptic number.

It is an amenable number.

21020420261 is a deficient number, since it is larger than the sum of its proper divisors (1).

21020420261 is an equidigital number, since it uses as much as digits as its factorization.

21020420261 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 384, while the sum is 20.

Adding to 21020420261 its reverse (16202402012), we get a palindrome (37222822273).

The spelling of 21020420261 in words is "twenty-one billion, twenty million, four hundred twenty thousand, two hundred sixty-one".