Stiv’s Diabolical Devices now gives a bundle of precisely 47 equal-length rods that may be joined by hinges at their ends – and solely the ends – to kind planar linkages (i.e. all hinge axes are regular to the airplane containing the rods and rod thickness is uncared for).
With 35 of those rods you can also make a inflexible linkage containing the vertices of an everyday heptagon:
Now you wish to do the identical for an everyday nonagon. However the perfect recognized bracing of an everyday nonagon makes use of 51 rods, greater than you will have from one bundle:
Nonetheless it’s doable to kind a inflexible linkage containing the vertices of an everyday nonagon utilizing solely 47 rods by imagining that you’ve got 7 extra rods, making a absolutely symmetric ($D_{18}$ symmetry, similar as that of the nonagon) inflexible linkage with these 54 rods and eradicating 7 redundant rods. What does the 47-rod linkage seem like?
Looking out my MathWorld contributions will assist (MathWorld Contributors -> Tan).
