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- #!/usr/bin/ruby
- # a(n) is the least positive number k such that 3^n + k is n-almost prime (first n-almost prime after 3^n).
- # https://oeis.org/A337219
- # Known terms:
- # 2, 1, 1, 3, 9, 7, 21, 63, 157, 471, 5, 15, 45, 135, 405, 1215, 3645, 10935, 32805, 98415, 295245, 885735, 2657205, 4409119, 2741597, 8224791, 16285765, 15302863, 45908589, 137725767, 77632981, 232898943, 161825917, 485477751, 1456433253, 3027122479, 1565174669, 4695524007, 14086572021
- func a(n) {
- 3**n -> next_almost_prime(n) - 3**n
- }
- for n in (1..100) {
- say "#{n} #{a(n)}"
- }
- __END__
- 1 2
- 2 1
- 3 1
- 4 3
- 5 9
- 6 7
- 7 21
- 8 63
- 9 157
- 10 471
- 11 5
- 12 15
- 13 45
- 14 135
- 15 405
- 16 1215
- 17 3645
- 18 10935
- 19 32805
- 20 98415
- 21 295245
- 22 885735
- 23 2657205
- 24 4409119
- 25 2741597
- 26 8224791
- 27 16285765
- 28 15302863
- 29 45908589
- 30 137725767
- 31 77632981
- 32 232898943
- 33 161825917
- 34 485477751
- 35 1456433253
- 36 3027122479
- 37 1565174669
- 38 4695524007
- 39 14086572021
- 40 25079846879
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