The (double) Primorial sieve & Prime spirals |
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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.