The (double) Primorial sieve  &  Prime spirals E-mail

The segmented prime spirals are an offspring of the Ulam spiral.
The counterclockwise Ulam spiral with startvalue  0  is a  four-quarter spiral,  and thus a prime spiral with four segments. Downwards is the prime spiral with  three  segments, which can be visualized as a  three-quarter  Ulam spiral.

The Ulam three-quarter spiral (see above) becomes a continuous spiral (see below) when the prime spiral is folded together. Clearly visible is the translation  n   to  n - 1  at the original seam.

The Ulam three-quarter spiral can also be presented as a hexagram, see the last two pictures..


Prime spirals: The Ulam three-quarter spiral as prime spiral with three segments.