## Investigating Gaussian Integers

### Problem 153

Published on Saturday, 5th May 2007, 10:00 am; Solved by 1342As we all know the equation `x`^{2}=-1 has no solutions for real `x`.

If we however introduce the imaginary number `i` this equation has two solutions: `x=i` and `x=-i`.

If we go a step further the equation (`x`-3)^{2}=-4 has two complex solutions: `x`=3+2`i` and `x`=3-2`i`.

`x`=3+2`i` and `x`=3-2`i` are called each others' complex conjugate.

Numbers of the form `a`+`bi` are called complex numbers.

In general `a`+`bi` and `a``bi` are each other's complex conjugate.

A Gaussian Integer is a complex number `a`+`bi` such that both `a` and `b` are integers.

The regular integers are also Gaussian integers (with `b`=0).

To distinguish them from Gaussian integers with `b` 0 we call such integers "rational integers."

A Gaussian integer is called a divisor of a rational integer `n` if the result is also a Gaussian integer.

If for example we divide 5 by 1+2`i` we can simplify in the following manner:

Multiply numerator and denominator by the complex conjugate of 1+2`i`: 12`i`.

The result is
.

So 1+2`i` is a divisor of 5.

Note that 1+`i` is not a divisor of 5 because .

Note also that if the Gaussian Integer (`a`+`bi`) is a divisor of a rational integer `n`, then its complex conjugate (`a``bi`) is also a divisor of `n`.

In fact, 5 has six divisors such that the real part is positive: {1, 1 + 2`i`, 1 2`i`, 2 + `i`, 2 `i`, 5}.

The following is a table of all of the divisors for the first five positive rational integers:

n | Gaussian integer divisors with positive real part | Sum s(n) of these divisors |

1 | 1 | 1 |

2 | 1, 1+i, 1-i, 2 | 5 |

3 | 1, 3 | 4 |

4 | 1, 1+i, 1-i, 2, 2+2i, 2-2i,4 | 13 |

5 | 1, 1+2i, 1-2i, 2+i, 2-i, 5 | 12 |

For divisors with positive real parts, then, we have: .

For 1 `n` 10^{5}, s(`n`)=17924657155.

What is s(`n`) for 1 `n` 10^{8}?