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- #!/usr/bin/ruby
- # a(n) is the index of the smallest tetrahedral number with exactly n prime factors (counted with multiplicity), or -1 if no such number exists.
- # https://oeis.org/A359090
- # Previously known terms:
- # 1, -1, 2, 4, 6, 8, 14, 30, 48, 62, 126, 160, 350, 510, 1022, 2046, 1024, 4095, 4094, 13310, 28672, 32768, 65534, 180224, 262142, 360448, 262143
- # New terms a(27)-a(34):
- # 2097151, 3276800, 4194302, 2097150, 33554432, 16777214, 66715648, 33554430
- #`(
- # PARI/GP program:
- a(n) = if(n==1, return(-1)); for(k=1, oo, my(t=(k*(k+1)*(k+2))\6); if(bigomega(t) == n, return(k))); \\ ~~~~
- )
- func a(n) {
- for k in (1..Inf) {
- if (pyramidal(k, 3).is_almost_prime(n)) {
- return k
- }
- }
- }
- for n in (2..100) {
- say "a(#{n}) = #{a(n)}"
- }
- __END__
- a(2) = 2
- a(3) = 4
- a(4) = 6
- a(5) = 8
- a(6) = 14
- a(7) = 30
- a(8) = 48
- a(9) = 62
- a(10) = 126
- a(11) = 160
- a(12) = 350
- a(13) = 510
- a(14) = 1022
- a(15) = 2046
- a(16) = 1024
- a(17) = 4095
- a(18) = 4094
- a(19) = 13310
- a(20) = 28672
- a(21) = 32768
- a(22) = 65534
- a(23) = 180224
- a(24) = 262142
- a(25) = 360448
- a(26) = 262143
- a(27) = 2097151
- a(28) = 3276800
- a(29) = 4194302
- a(30) = 2097150
- a(31) = 33554432
- a(32) = 16777214
- a(33) = 66715648
- a(34) = 33554430
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