21161556 has 72 divisors (see below), whose sum is σ = 58298240. Its totient is φ = 6433344.

The previous prime is 21161551. The next prime is 21161557. The reversal of 21161556 is 65516112.

It is a happy number.

It is a super-2 number, since 2×21161556² = 895622904682272, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21161551) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 47985 + ... + 48423.

Almost surely, 2^21161556 is an apocalyptic number.

21161556 is a gapful number since it is divisible by the number (26) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21161556, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (29149120).

21161556 is an abundant number, since it is smaller than the sum of its proper divisors (37136684).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21161556 is a wasteful number, since it uses less digits than its factorization.

21161556 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 565 (or 560 counting only the distinct ones).

The product of its digits is 1800, while the sum is 27.

The square root of 21161556 is about 4600.1691273256. The cubic root of 21161556 is about 276.5981043312.

Adding to 21161556 its reverse (65516112), we get a palindrome (86677668).

The spelling of 21161556 in words is "twenty-one million, one hundred sixty-one thousand, five hundred fifty-six".