20991556 has 6 divisors (see below), whose sum is σ = 36735230. Its totient is φ = 10495776.

The previous prime is 20991533. The next prime is 20991577. The reversal of 20991556 is 65519902.

It can be written as a sum of positive squares in only one way, i.e., 12960000 + 8031556 = 3600^2 + 2834^2 .

It is a super-2 number, since 2×20991556² = 881290846602272, which contains 22 as substring.

It is an unprimeable number.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 2623941 + ... + 2623948.

Almost surely, 2^20991556 is an apocalyptic number.

It is an amenable number.

20991556 is a deficient number, since it is larger than the sum of its proper divisors (15743674).

20991556 is a wasteful number, since it uses less digits than its factorization.

20991556 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5247893 (or 5247891 counting only the distinct ones).

The product of its (nonzero) digits is 24300, while the sum is 37.

The square root of 20991556 is about 4581.6542863905. The cubic root of 20991556 is about 275.8554343383.

It can be divided in two parts, 2099 and 1556, that added together give a triangular number (3655 = T₈₅).

The spelling of 20991556 in words is "twenty million, nine hundred ninety-one thousand, five hundred fifty-six".