← Complete problem index

PROJECT EULER · #0295

Lenticular Holes

Statement only · UnsolvedOriginal problem ↗

We call the convex area enclosed by two circles a lenticular hole if:

  • The centres of both circles are on lattice points.
  • The two circles intersect at two distinct lattice points.
  • The interior of the convex area enclosed by both circles does not contain any lattice points.

Consider the circles:
C0: x2+y2=25
C1: (x+4)2+(y4)2=1
C2: (x12)2+(y4)2=65

The circles C0, C1 and C2 are drawn in the picture below.

0295_lenticular.gif

C0 and C1 form a lenticular hole, as well as C0 and C2.

We call an ordered pair of positive real numbers (r1,r2) a lenticular pair if there exist two circles with radii r1 and r2 that form a lenticular hole. We can verify that (1,5) and (5,65) are the lenticular pairs of the example above.

Let L(N) be the number of distinct lenticular pairs (r1,r2) for which 0<r1r2N.
We can verify that L(10)=30 and L(100)=3442.

Find L(100000).

Write-up coming later

The complete problem is available here. An approach, code, and answer will be added later.