Fermat pseudoprimes

Fermat pseudoprimes is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

A Fermat pseudoprime is a positive composite integer that fails the Fermat primality test.

Fermat's little theorem states that if p is prime and a is coprime to p, then a^p−1 − 1 is divisible by p.

For an integer a > 1, if a composite integer x divides a^x−1 − 1, then x is called a Fermat pseudoprime to base a.

Fermat pseudoprimes to base 2 are sometimes called Sarrus numbers or Poulet numbers.

Fermat pseudoprimes can be found to any positive integer base. When using a base integer a = 1, this method returns all composite numbers.

Task

For base integers a of 1 through 20:

• Find the count of pseudoprimes up to and including 12,000.
• Show the first 20 pseudoprimes.

Stretch

• Extend the count threshold out to 25,000 or 50,000.

See also

• Wikipedia: Fermat pseudoprime
• OEIS:A002808 - Composite numbers
• OEIS:A001567 - Fermat pseudoprimes to base 2
• OEIS:A005935 - Fermat pseudoprimes to base 3
• OEIS:A020136 - Fermat pseudoprimes to base 4
• OEIS:A005936 - Fermat pseudoprimes to base 5
• OEIS:A005937 - Fermat pseudoprimes to base 6
• OEIS:A005938 - Fermat pseudoprimes to base 7
• OEIS:A020137 - Fermat pseudoprimes to base 8
• OEIS:A020138 - Fermat pseudoprimes to base 9
• OEIS:A005939 - Fermat pseudoprimes to base 10
• OEIS:A020139 - Fermat pseudoprimes to base 11
• OEIS:A020140 - Fermat pseudoprimes to base 12
• OEIS:A020141 - Fermat pseudoprimes to base 13
• OEIS:A020142 - Fermat pseudoprimes to base 14
• OEIS:A020143 - Fermat pseudoprimes to base 15
• OEIS:A020144 - Fermat pseudoprimes to base 16
• OEIS:A020145 - Fermat pseudoprimes to base 17
• OEIS:A020146 - Fermat pseudoprimes to base 18
• OEIS:A020147 - Fermat pseudoprimes to base 19
• OEIS:A020148 - Fermat pseudoprimes to base 20

Raku

<lang perl6>use List::Divvy; for 1..20 -> $base {

   my @pseudo = lazy (1..*).hyper.grep: { !.is-prime && (exp($_ - 1, $base) % $_ == 1) }
   my $threshold = 50000;
   say $base.fmt("Base %2d - Up to $threshold: " ~ (+@pseudo.&upto: $threshold).fmt('%5d')
       ~ "  First 20: ") ~ @pseudo[^20].gist

}</lang>

Base  1 - Up to 50000: 44866  First 20: (4 6 8 9 10 12 14 15 16 18 20 21 22 24 25 26 27 28 30 32)
Base  2 - Up to 50000:    55  First 20: (341 561 645 1105 1387 1729 1905 2047 2465 2701 2821 3277 4033 4369 4371 4681 5461 6601 7957 8321)
Base  3 - Up to 50000:    53  First 20: (91 121 286 671 703 949 1105 1541 1729 1891 2465 2665 2701 2821 3281 3367 3751 4961 5551 6601)
Base  4 - Up to 50000:   111  First 20: (15 85 91 341 435 451 561 645 703 1105 1247 1271 1387 1581 1695 1729 1891 1905 2047 2071)
Base  5 - Up to 50000:    54  First 20: (4 124 217 561 781 1541 1729 1891 2821 4123 5461 5611 5662 5731 6601 7449 7813 8029 8911 9881)
Base  6 - Up to 50000:    74  First 20: (35 185 217 301 481 1105 1111 1261 1333 1729 2465 2701 2821 3421 3565 3589 3913 4123 4495 5713)
Base  7 - Up to 50000:    49  First 20: (6 25 325 561 703 817 1105 1825 2101 2353 2465 3277 4525 4825 6697 8321 10225 10585 10621 11041)
Base  8 - Up to 50000:   150  First 20: (9 21 45 63 65 105 117 133 153 231 273 341 481 511 561 585 645 651 861 949)
Base  9 - Up to 50000:   113  First 20: (4 8 28 52 91 121 205 286 364 511 532 616 671 697 703 946 949 1036 1105 1288)
Base 10 - Up to 50000:    65  First 20: (9 33 91 99 259 451 481 561 657 703 909 1233 1729 2409 2821 2981 3333 3367 4141 4187)
Base 11 - Up to 50000:    61  First 20: (10 15 70 133 190 259 305 481 645 703 793 1105 1330 1729 2047 2257 2465 2821 4577 4921)
Base 12 - Up to 50000:    91  First 20: (65 91 133 143 145 247 377 385 703 1045 1099 1105 1649 1729 1885 1891 2041 2233 2465 2701)
Base 13 - Up to 50000:    68  First 20: (4 6 12 21 85 105 231 244 276 357 427 561 1099 1785 1891 2465 2806 3605 5028 5149)
Base 14 - Up to 50000:    69  First 20: (15 39 65 195 481 561 781 793 841 985 1105 1111 1541 1891 2257 2465 2561 2665 2743 3277)
Base 15 - Up to 50000:    42  First 20: (14 341 742 946 1477 1541 1687 1729 1891 1921 2821 3133 3277 4187 6541 6601 7471 8701 8911 9073)
Base 16 - Up to 50000:   145  First 20: (15 51 85 91 255 341 435 451 561 595 645 703 1105 1247 1261 1271 1285 1387 1581 1687)
Base 17 - Up to 50000:    63  First 20: (4 8 9 16 45 91 145 261 781 1111 1228 1305 1729 1885 2149 2821 3991 4005 4033 4187)
Base 18 - Up to 50000:    98  First 20: (25 49 65 85 133 221 323 325 343 425 451 637 931 1105 1225 1369 1387 1649 1729 1921)
Base 19 - Up to 50000:    93  First 20: (6 9 15 18 45 49 153 169 343 561 637 889 905 906 1035 1105 1629 1661 1849 1891)
Base 20 - Up to 50000:    66  First 20: (21 57 133 231 399 561 671 861 889 1281 1653 1729 1891 2059 2413 2501 2761 2821 2947 3059)