cp's OEIS Frontend

A265784 Numerators of lower primes-only best approximates (POBAs) to sqrt(5); see Comments.

%I A265784 #10 Apr 06 2019 01:09:30
%S A265784 3,5,11,29,163,199,521,3571,91283,150427
%N A265784 Numerators of lower primes-only best approximates (POBAs) to sqrt(5); see Comments.
%C A265784 Suppose that x > 0. A fraction p/q of primes is a lower primes-only best approximate, and we write "p/q is in L(x)", if u/v < p/q < x < p'/q for all primes u and v such that v < q, where p' is least prime > p.
%C A265784 Let q(1) be the least prime q such that u/q < x for some prime u, and let p(1) be the greatest such u. The sequence L(x) follows inductively: for n > 1, let q(n) is the least prime q such that p(n)/q(n) < p/q < x for some prime p. Let q(n+1) = q and let p(n+1) be the greatest prime p such that p(n)/q(n) < p/q < x.
%C A265784 For a guide to POBAs, lower POBAs, and upper POBAs, see A265759.
%e A265784 The lower POBAs to sqrt(5) start with 3/2, 5/3, 11/5, 29/13, 163/73, 199/89, 521/233. For example, if p and q are primes and q > 73, and p/q < sqrt(5), then 163/73 is closer to sqrt(5) than p/q is.
%t A265784 x = Sqrt[5]; z = 1000; p[k_] := p[k] = Prime[k];
%t A265784 t = Table[Max[Table[NextPrime[x*p[k], -1]/p[k], {k, 1, n}]], {n, 1, z}];
%t A265784 d = DeleteDuplicates[t]; tL = Select[d, # > 0 &] (* lower POBA *)
%t A265784 t = Table[Min[Table[NextPrime[x*p[k]]/p[k], {k, 1, n}]], {n, 1, z}];
%t A265784 d = DeleteDuplicates[t]; tU = Select[d, # > 0 &] (* upper POBA *)
%t A265784 v = Sort[Union[tL, tU], Abs[#1 - x] > Abs[#2 - x] &];
%t A265784 b = Denominator[v]; s = Select[Range[Length[b]], b[[#]] == Min[Drop[b, # - 1]] &];
%t A265784 y = Table[v[[s[[n]]]], {n, 1, Length[s]}] (* POBA, A265782/A265783 *)
%t A265784 Numerator[tL]   (* A265784 *)
%t A265784 Denominator[tL] (* A265785 *)
%t A265784 Numerator[tU]   (* A265786 *)
%t A265784 Denominator[tU] (* A265787 *)
%t A265784 Numerator[y]    (* A222588 *)
%t A265784 Denominator[y]  (* A265789 *)
%Y A265784 Cf. A000040, A265759, A265785, A265786, A265787, A265788, A265789.
%K A265784 nonn,frac,more
%O A265784 1,1
%A A265784 _Clark Kimberling_, Dec 23 2015
%E A265784 a(9)-a(10) from _Robert Price_, Apr 05 2019