Number of the day: 4991

Properties of the number 4991:

4991 = 7 × 23 × 31 is a sphenic number and squarefree.
4991 has 3 distinct prime factors, 8 divisors, 13 antidivisors and 3960 totatives.
4991 has a prime digit sum 23 in base 10.
Reversing the decimal digits of 4991 results in a semiprime.
4991 = 2496² - 2495² = 360² - 353² = 120² - 97² = 96² - 65² is the difference of 2 nonnegative squares in 4 ways.
4991 is the difference of 2 positive pentagonal numbers in 3 ways.
4991 is not the sum of 3 positive squares.
4991² is the sum of 3 positive squares.
4991 is a divisor of 1427² - 1.
4991 = '49' + '91' is the concatenation of 2 semiprime numbers.
4991 is palindromic in (at least) the following bases: 19, -28, and -43.
4991 in base 19 = dfd and consists of only the digits 'd' and 'f'.

The number 4991 belongs to the following On-Line Encyclopedia of Integer Sequences (OEIS) sequences (among others):

Sequence numbers and descriptions below are taken from OEIS.
A001845: Centered octahedral numbers (crystal ball sequence for cubic lattice).
A004006: C(n,1) + C(n,2) + C(n,3), or n*(n^2+5)/6.
A005008: n! - n^2.
A006972: Lucas-Carmichael numbers: squarefree composite numbers n such that p | n => p+1 | n+1.
A152942: Odd squarefree numbers n such that the cyclotomic polynomial Phi(n,x) has height 4.
A195575: Numerators b(n) of Pythagorean approximations b(n)/a(n) to 2/5.
A199118: Number of partitions of n into terms of (1,3)-Ulam sequence, cf. A002859.
A216925: Lucas-Carmichael numbers with 3 prime factors.
A257752: Quasi-Carmichael numbers to exactly two bases.
A263799: T(n,k)=Number of (n+1)X(k+1) 0..1 arrays with each row and column divisible by 3, read as a binary number with top and left being the most significant bits, and rows and columns lexicographically nonincreasing