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

2, 5, 7, 13, 59, 199, 257, 397, 467, 653, 991, 1597, 16273, 28163, 30133, 36451, 64579, 70489

Offset

1,1

Comments

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.

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).

For a guide to POBAs, lower POBAs, and upper POBAs, see A265759.

Links

Table of n, a(n) for n=1..18.

Example

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.

Mathematica

x = Sqrt[8]; z = 1000; p[k_] := p[k] = Prime[k];
t = Table[Max[Table[NextPrime[x*p[k], -1]/p[k], {k, 1, n}]], {n, 1, z}];
d = DeleteDuplicates[t]; tL = Select[d, # > 0 &] (* lower POBA *)
t = Table[Min[Table[NextPrime[x*p[k]]/p[k], {k, 1, n}]], {n, 1, z}];
d = DeleteDuplicates[t]; tU = Select[d, # > 0 &] (* upper POBA *)
v = Sort[Union[tL, tU], Abs[#1 - x] > Abs[#2 - x] &];
b = Denominator[v]; s = Select[Range[Length[b]], b[[#]] == Min[Drop[b, # - 1]] &];
y = Table[v[[s[[n]]]], {n, 1, Length[s]}] (* POBA, A265794/A265795 *)
Numerator[tL]   (* A265790 *)
Denominator[tL] (* A265791 *)
Numerator[tU]   (* A265792 *)
Denominator[tU] (* A265793 *)
Numerator[y]    (* A265794 *)
Denominator[y]  (* A265795 *)

Crossrefs

Cf. A000040, A265759, A265790, A265791, A265792, A265794, A265795.

Sequence in context: A164570 A348671 A265811 * A126338 A305993 A193718

Adjacent sequences: A265790 A265791 A265792 * A265794 A265795 A265796

Keyword

nonn,frac,more

Author

Clark Kimberling, Dec 29 2015

Extensions

a(13)-a(18) from Robert Price, Apr 06 2019

Status

approved