cp's OEIS Frontend

A265793 Denominators of upper primes-only best approximates (POBAs) to sqrt(8); see Comments.

%I A265793 #10 Apr 06 2019 12:51:17
%S A265793 2,5,7,13,59,199,257,397,467,653,991,1597,16273,28163,30133,36451,
%T A265793 64579,70489
%N A265793 Denominators of upper primes-only best approximates (POBAs) to sqrt(8); see Comments.
%C A265793 Suppose that x > 0. A fraction p/q of primes is an upper primes-only best approximate, and we write "p/q is in U(x)", if p'/q < x < p/q < u/v for all primes u and v such that v < q, where p' is greatest prime < p in case p >= 3.
%C A265793 Let q(1) = 2 and let p(1) be the least prime >= x. The sequence U(x) follows inductively: for n >= 1, let q(n) is the least prime q such that x < p/q < p(n)/q(n) for some prime p. Let q(n+1) = q and let p(n+1) be the least prime p such that x < p/q < p(n)/q(n).
%C A265793 For a guide to POBAs, lower POBAs, and upper POBAs, see A265759.
%e A265793 The upper POBAs to sqrt(8) start with 7/2, 17/5, 23/7, 37/13, 167/59, 563/199, 727/257, 1123/397. For example, if p and q are primes and q > 13, and p/q > sqrt(8), then 37/13 is closer to sqrt(8) than p/q is.
%t A265793 x = Sqrt[8]; z = 1000; p[k_] := p[k] = Prime[k];
%t A265793 t = Table[Max[Table[NextPrime[x*p[k], -1]/p[k], {k, 1, n}]], {n, 1, z}];
%t A265793 d = DeleteDuplicates[t]; tL = Select[d, # > 0 &] (* lower POBA *)
%t A265793 t = Table[Min[Table[NextPrime[x*p[k]]/p[k], {k, 1, n}]], {n, 1, z}];
%t A265793 d = DeleteDuplicates[t]; tU = Select[d, # > 0 &] (* upper POBA *)
%t A265793 v = Sort[Union[tL, tU], Abs[#1 - x] > Abs[#2 - x] &];
%t A265793 b = Denominator[v]; s = Select[Range[Length[b]], b[[#]] == Min[Drop[b, # - 1]] &];
%t A265793 y = Table[v[[s[[n]]]], {n, 1, Length[s]}] (* POBA, A265794/A265795 *)
%t A265793 Numerator[tL]   (* A265790 *)
%t A265793 Denominator[tL] (* A265791 *)
%t A265793 Numerator[tU]   (* A265792 *)
%t A265793 Denominator[tU] (* A265793 *)
%t A265793 Numerator[y]    (* A265794 *)
%t A265793 Denominator[y]  (* A265795 *)
%Y A265793 Cf. A000040, A265759, A265790, A265791, A265792, A265794, A265795.
%K A265793 nonn,frac,more
%O A265793 1,1
%A A265793 _Clark Kimberling_, Dec 29 2015
%E A265793 a(13)-a(18) from _Robert Price_, Apr 06 2019