A265795 - OEIS

%I #14 Apr 06 2019 12:50:49

%S 2,2,5,7,11,59,127,163,233,653,991,1597,11447,12671,70489

%N Denominators of primes-only best approximates (POBAs) to sqrt(8); 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(8) start with 7/2, 5/2, 13/5, 19/7, 31/11, 167/59, 359/127, 461/163, 659/233. For example, if p and q are primes and q > 59, then 167/59 is closer to sqrt(8) than p/q is.

%t x = Sqrt[8]; 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, A265794/A265795 *)

%t Numerator[tL]   (* A265790 *)

%t Denominator[tL] (* A265791 *)

%t Numerator[tU]   (* A265792 *)

%t Denominator[tU] (* A265793 *)

%t Numerator[y]    (* A265794 *)

%t Denominator[y]  (* A265795 *)

%Y Cf. A000040, A010466, A265759, A265790, A265791, A265792, A265793, A265794.

%K nonn,frac,more

%O 1,1

%A _Clark Kimberling_, Dec 29 2015

%E a(13)-a(15) from _Robert Price_, Apr 06 2019