A308220 - OEIS

A308220

Number of irreducible Egyptian fractions of denominator n which are the sum of 2 unit fractions.

2, 2, 3, 3, 4, 3, 4, 4, 5, 4, 6, 4, 4, 5, 5, 5, 6, 5, 6, 6, 8, 4, 8, 6, 6, 7, 8, 7, 8, 6, 6, 7, 7, 7, 10, 7, 4, 7, 11, 8, 8, 7, 6, 8, 9, 5, 10, 9, 8, 9, 9, 9, 8, 7, 12, 9, 10, 6, 12, 9, 4, 8, 11, 9, 12, 9, 6, 10, 12, 8, 12, 10, 4, 9, 13, 10, 13, 7, 10, 11, 10, 6, 12, 12, 10, 9, 12, 10, 12, 10, 10, 9, 10, 7, 12, 11, 6, 11, 13, 12

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OFFSET

1,1

LINKS

Table of n, a(n) for n=1..100.

Cyril Banderier, Florian Luca, Francesco Pappalardi, Numerators of Egyptian fractions, 2019.

FORMULA

For p prime, a(p) = number_of_divisors(p+1).

EXAMPLE

There are a(2)=2 irreducible fractions with denominator n=2 which are sums of 2 unit fractions: 1/2 = 1/4 + 1/4 and 3/2 = 1/1 + 1/2.

CROSSREFS

Cf. A308219, A308221, A308415.