cp's OEIS Frontend

A265801 Denominators of primes-only best approximates (POBAs) to the golden ratio, tau; see Comments.

%I A265801 #15 Apr 07 2019 00:02:17
%S A265801 2,2,3,7,19,23,29,97,353,563,631,919,1453,2207,15271,15737,42797,49939
%N A265801 Denominators of primes-only best approximates (POBAs) to the golden ratio, tau; see Comments.
%C A265801 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.
%C A265801 Is this related to A165571? - _R. J. Mathar_, Jan 10 2016
%e A265801 The POBAs to tau start with 5/2, 3/2, 5/3, 11/7, 31/19, 37/23, 47/29, 157/97, 571/353, 911/563. For example, if p and q are primes and q > 29, then 47/29 is closer to tau than p/q is.
%t A265801 x = GoldenRatio; z = 1000; p[k_] := p[k] = Prime[k];
%t A265801 t = Table[Max[Table[NextPrime[x*p[k], -1]/p[k], {k, 1, n}]], {n, 1, z}];
%t A265801 d = DeleteDuplicates[t]; tL = Select[d, # > 0 &] (* lower POBA *)
%t A265801 t = Table[Min[Table[NextPrime[x*p[k]]/p[k], {k, 1, n}]], {n, 1, z}];
%t A265801 d = DeleteDuplicates[t]; tU = Select[d, # > 0 &] (* upper POBA *)
%t A265801 v = Sort[Union[tL, tU], Abs[#1 - x] > Abs[#2 - x] &];
%t A265801 b = Denominator[v]; s = Select[Range[Length[b]], b[[#]] == Min[Drop[b, # - 1]] &];
%t A265801 y = Table[v[[s[[n]]]], {n, 1, Length[s]}] (* POBA, A265800/A265801 *)
%t A265801 Numerator[tL]   (* A265796 *)
%t A265801 Denominator[tL] (* A265797 *)
%t A265801 Numerator[tU]   (* A265798 *)
%t A265801 Denominator[tU] (* A265799 *)
%t A265801 Numerator[y]    (* A265800 *)
%t A265801 Denominator[y]  (* A265801 *)
%Y A265801 Cf. A000040, A265759, A265796, A265797, A265798, A265799, A265800.
%K A265801 nonn,frac,more
%O A265801 1,1
%A A265801 _Clark Kimberling_, Jan 02 2016
%E A265801 a(15)-a(18) from _Robert Price_, Apr 06 2019