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# Zeisel numbers

The Zeisel numbers are squarefree numbers with at least three prime factors constructed in the following way: to construct a Zeisel number, you start with $p_{0}\,=\,1\,$ and two integers $a\,$ and $b\,$ . All numbers of the form $p_{n}\,=\,ap_{n-1}+b\,$ with $n\,\geq \,1\,$ have to be prime numbers.

The Zeisel numbers are named after the austrian mathematician Helmut Zeisel.

Examples:

$a=1,\,b=6:\ p_{1}=7,\,p_{2}=13,\,p_{3}=19\,$ => $1729=7\cdot 13\cdot 19\,$ $a=4,\,b=3:\ p_{1}=7,\,p_{2}=31,\,p_{3}=127\,$ => $27559=7\cdot 31\cdot 127\,$ $a=8,\,b=-3:\ p_{1}=5,\,p_{2}=37,\,p_{3}=293\,$ => $54205=5\cdot 37\cdot 293\,$ ## Chernick Carmichael numbers and Zeisel numbers

Every Chernick Carmichael number is a Zeisel number with $a\,=\,1\,$ and $b\,=\,6n\,$ .

## Generalization of the Zeisel numbers

It is possible to use a $p_{0}\,$ dfferent from $1\,$ Examples:

• $p_{0}=4,\ a=2,\ b=5$ p0 = 4
p1 = a·p0 + b = 2·4  + 5 = 13
p2 = a·p1 + b = 2·13 + 5 = 31
p3 = a·p2 + b = 2·31 + 5 = 67

z = p1 · p2 · p3 = 13 · 31 · 67 = 27001

• $p_{0}=-1,\ a=8,\ b=27$ p0 = -1
p1 = a·p0 + b = 8·-1   + 27 =   19
p2 = a·p1 + b = 8·19   + 27 =  179
p3 = a·p2 + b = 28·179 + 27 = 1459

z = p1 · p2 · p3 = 19 · 179 · 1459 = 4962059


## Zeisel numbers and Fermat pseudoprimes

Every Zeisel number $n\,$ is a Fermat pseudoprime to some base $b\,$ .

## Sequences

### Zeisel numbers related sequences

The Zeisel numbers (Cf. A051015) are

{105, 1419, 1729, 1885, 4505, 5719, 15387, 24211, 25085, 27559, 31929, 54205, 59081, 114985, 207177, 208681, 233569, 287979, 294409, 336611, 353977, 448585, 507579, 721907, 982513, ...}

The extended Chernick Carmichael numbers (Cf. AXXXXXX) are

{1729, 63973, 294409, ...}