next_almost_prime.sf

#!/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