A265789 - OEIS

%I #10 Apr 06 2019 01:09:13

%S 2,2,3,5,13,73,89,233,1597,11933,49939,67273,69247

%N Denominators of primes-only best approximates (POBAs) to sqrt(5); see Comments.

%C Suppose that x > 0. A fraction p/q of primes is a primes-only best approximate (POBA), and we write "p/q in B(x)", if 0 < |x - p/q| < |x - u/v| for all primes u and v such that v < q, and also, |x - p/q| < |x - p'/q| for every prime p' except p. Note that for some choices of x, there are values of q for which there are two POBAs. In these cases, the greater is placed first; e.g., B(3) = (7/2, 5/2, 17/5, 13/5, 23/7, 19/7, ...). See A265759 for a guide to related sequences.

%e The POBAs to sqrt(5) start with 3/2, 5/2, 7/3, 11/5, 29/13, 163/73, 199/89, 521/233. For example, if p and q are primes and q > 89, then 199/89 is closer to sqrt(5) than p/q is.

%t x = Sqrt[5]; z = 1000; p[k_] := p[k] = Prime[k];

%t t = Table[Max[Table[NextPrime[x*p[k], -1]/p[k], {k, 1, n}]], {n, 1, z}];

%t d = DeleteDuplicates[t]; tL = Select[d, # > 0 &] (* lower POBA *)

%t t = Table[Min[Table[NextPrime[x*p[k]]/p[k], {k, 1, n}]], {n, 1, z}];

%t d = DeleteDuplicates[t]; tU = Select[d, # > 0 &] (* upper POBA *)

%t v = Sort[Union[tL, tU], Abs[#1 - x] > Abs[#2 - x] &];

%t b = Denominator[v]; s = Select[Range[Length[b]], b[[#]] == Min[Drop[b, # - 1]] &];

%t y = Table[v[[s[[n]]]], {n, 1, Length[s]}] (* POBA, A265782/A265783 *)

%t Numerator[tL]   (* A265784 *)

%t Denominator[tL] (* A265785 *)

%t Numerator[tU]   (* A265786 *)

%t Denominator[tU] (* A265787 *)

%t Numerator[y]    (* A265788 *)

%t Denominator[y]  (* A265789 *)

%Y Cf. A000040, A265759, A265784, A265785, A265786, A265788, A265789.

%K nonn,frac,more

%O 1,1

%A _Clark Kimberling_, Dec 29 2015

%E a(10)-a(13) from _Robert Price_, Apr 05 2019