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- #!/usr/bin/ruby
- # Indices k such that A002533(k) is prime.
- # https://oeis.org/A372491
- # Known terms:
- # 2, 3, 4, 5, 11, 17, 32, 53, 103, 107, 109, 113, 137, 197, 233, 811, 7993, 9281, 14387, 26573, 51361
- # It's very hard to find the next term, as most terms do not have small factors,
- # therefore we need to run a pseudoprime test on most numbers, which each take several seconds.
- Num!VERBOSE = true
- var from = 51481
- for k in (from..1e9) {
- k.is_prime || k.is_power_of(2) || next
- var v = Math.linear_rec([2,5], [1,1], k)
- say ":: Testing: #{k} (length: #{v.len})"
- if (v.is_prob_prime) {
- die "\nFound new term: #{k}\n"
- }
- }
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