Number of the day: 7978

Properties of the number 7978:

7978 = 2 × 3989 is semiprime and squarefree.
7978 has 2 distinct prime factors, 4 divisors, 11 antidivisors and 3988 totatives.
7978 has an emirp digit sum 31 in base 10.
Reversing the decimal digits of 7978 results in an emirpimes.
7978 is the difference of 2 positive pentagonal numbers in 2 ways.
7978 = 33² + 83² is the sum of 2 positive squares in 1 way.
7978 = 3² + 20² + 87² is the sum of 3 positive squares.
7978² = 5478² + 5800² is the sum of 2 positive squares in 1 way.
7978² is the sum of 3 positive squares.
7978 is a proper divisor of 29⁹⁹⁷ - 1.
7978 is an emirpimes in (at least) the following bases: 3, 4, 8, 10, 11, 14, 15, 19, 23, 25, 26, 27, 28, 30, 33, 34, 37, 41, 47, 50, 52, 53, 55, 56, 60, 62, 66, 68, 73, 74, 76, 77, 80, 81, 82, 85, 88, 89, 91, 92, and 96.
7978 is palindromic in (at least) the following bases: -21, and -55.
7978 in base 20 = jii and consists of only the digits 'i' and 'j'.
7978 in base 51 = 33M and consists of only the digits '3' and 'M'.

The number 7978 belongs to the following On-Line Encyclopedia of Integer Sequences (OEIS) sequences (among others):

Sequence numbers and descriptions below are taken from OEIS.
A031418: Numbers n such that continued fraction for sqrt(n) has odd period and a pair of central terms both equal to 5.
A036063: Increasing gaps among twin primes: size.
A041334: Numerators of continued fraction convergents to sqrt(181).
A057968: Triangle T(n,k) of numbers of minimal 5-covers of an unlabeled n+5-set that cover k points of that set uniquely (k=5,..,n+5).
A107853: G.f. x*(x-1)*(x+1)^3/((2*x^3+x^2-1)*(x^4+1)).
A152231: Similar to A072921 but starting with 2.
A224492: Smallest k such that k*2*p(n)^2-1=q is prime, k*2*q^2-1=r, k*2*r^2-1=s, k*2*r^2-1=t, r, s, and t are also prime.
A239469: Number of 3-separable partitions of n; see Comments.
A284068: Numbers whose smallest decimal digit is 7.
A304834: a(n) = 36*n^2 - 8*n - 2 (n >=1).