cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

Showing 1-4 of 4 results.

A265800 Numerators of primes-only best approximates (POBAs) to the golden ratio, tau; see Comments.

Original entry on oeis.org

5, 3, 5, 11, 31, 37, 47, 157, 571, 911, 1021, 1487, 2351, 3571, 24709, 25463, 69247, 80803
Offset: 1

Views

Author

Clark Kimberling, Dec 29 2015

Keywords

Comments

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.
How is this related to A165572? - R. J. Mathar, Jan 10 2016

Examples

			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.

Crossrefs

Cf. A000040, A265759, A265796, A265797, A265798, A265799, A265801.

Programs

  • Mathematica
    x = GoldenRatio; 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, A265800/A265801 *)
Numerator[tL]   (* A265796 *)
Denominator[tL] (* A265797 *)
Numerator[tU]   (* A265798 *)
Denominator[tU] (* A265799 *)
Numerator[y]    (* A265800 *)
Denominator[y]  (* A265801 *)

Extensions

a(15)-a(18) from Robert Price, Apr 06 2019

A165571 Lesser prime factor of successively better golden semiprimes.

Original entry on oeis.org

2, 3, 7, 19, 23, 29, 97, 353, 563, 631, 919, 1453, 2207, 15271, 15737, 42797, 49939, 133559, 179317, 287557, 508451, 918011, 1103483, 1981891, 9181097, 16958611, 17351447, 52204391, 66602803, 99641617, 134887397, 487195147, 629449511, 943818943, 1527963169, 2048029369
Offset: 1

Views

Author

Antti Karttunen, Sep 22 2009

Keywords

Comments

See A165569 and A165570 for the definition. Probably a subset of A108541.

Links

Crossrefs

Cf. A108539, A165569, A165570, A165572.

Programs

  • Mathematica
    f[p_] := Module[{x = GoldenRatio * p, p1, p2}, p1 = NextPrime[x, -1]; p2 = NextPrime[p1]; If[p2 - x > x - p1, p1, p2]]; seq={}; dm = 1; p1 = 1; Do[p1 = NextPrime[p1]; k++; p2 = f[p1]; d = Abs[p2/p1 - GoldenRatio]; If[d < dm, dm = d; AppendTo[seq, p1]], {10^4}]; seq  (* Amiram Eldar, Nov 28 2019 *)

Formula

a(n) = A000040(A165569(n)).
a(n) = A165570(n)/A165572(n).

Extensions

a(16)-a(23) from Donovan Johnson, May 13 2010
a(24)-a(36) from Amiram Eldar, Nov 28 2019

A165569 The indexing sequence for successively better golden semiprimes.

Original entry on oeis.org

1, 2, 4, 8, 9, 10, 25, 71, 103, 115, 157, 231, 329, 1783, 1835, 4476, 5128, 12462, 16274, 25035, 42174, 72589, 85968, 147666, 613726, 1088825, 1112415, 3125316, 3929736, 5742036, 7639447, 25716100, 32780150, 48132247, 76049401, 100464259, 108803364, 186018939
Offset: 1

Views

Author

Antti Karttunen, Sep 22 2009

Keywords

Links

Crossrefs

The corresponding semiprimes are given by A165570(n) = A165571(n)*A165572(n).
Cf. A108539.

Programs

  • Mathematica
    f[p_] := Module[{x = GoldenRatio * p, p1, p2}, p1 = NextPrime[x, -1]; p2 = NextPrime[p1]; If[p2 - x > x - p1, p1, p2]]; seq={}; k=0; dm = 1; p1 = 1; Do[p1 = NextPrime[p1]; k++; p2 = f[p1]; d = Abs[p2/p1 - GoldenRatio]; If[d < dm, dm = d; AppendTo[seq, k]], {10^4}]; seq (* Amiram Eldar, Nov 28 2019 *)

Formula

a(1)=1, and for n>1, a(n) = first such i>a(n-1) that abs(phi - A108539(i)/A000040(i)) < abs(phi - A108539(a(n-1))/A000040(a(n-1))), where phi = (1+sqrt(5))/2 (Golden ratio).

Extensions

a(16)-a(38) from Amiram Eldar, Nov 28 2019

A165570 Successively better golden semiprimes.

Original entry on oeis.org

6, 15, 77, 589, 851, 1363, 15229, 201563, 512893, 644251, 1366553, 3416003, 7881197, 377331139, 400711231, 2963563859, 4035221017, 28862500577, 52027213697, 133793658289, 418298061641, 1363588753103, 1970239102459, 6355462656397, 136388198153719, 465337655023099
Offset: 1

Views

Author

Antti Karttunen, Sep 22 2009

Keywords

Comments

This is lexicographically earliest sequence of such semiprimes p*q, starting from 6=2*3, that for each successive term p*q, q/p is a better approximant of Golden ratio (1+sqrt(5))/2 than the previous term. See A165569 for the exact procedure.
Can it be proved that this a subset of A108540?
The ratio A165572(n)/A165571(n) converges towards golden ratio = (1+sqrt(5))/2 = 1.618033988749895... as: 1.5, 1.6666666666666667, 1.5714285714285714, 1.631578947368421, 1.608695652173913, 1.6206896551724137, 1.6185567010309279, 1.6175637393767706, 1.6181172291296626, 1.618066561014263, 1.618063112078346, 1.618031658637302, 1.6180335296782964, 1.6180341824372995, 1.6180339327699054, ...

Links

Crossrefs

Cf. A108539, A165569, A165571, A165572.

Programs

  • Mathematica
    f[p_] := Module[{x = GoldenRatio * p, p1, p2}, p1 = NextPrime[x, -1]; p2 = NextPrime[p1]; If[p2 - x > x - p1, p1, p2]]; seq={}; dm = 1; p1 = 1; Do[p1 = NextPrime[p1]; k++; p2 = f[p1]; d = Abs[p2/p1 - GoldenRatio]; If[d < dm, dm = d; AppendTo[seq, p1*p2]], {10^4}]; seq (* Amiram Eldar, Nov 28 2019 *)

Formula

a(n) = A165571(n)*A165572(n) = A000040(A165569(n))*A108539(A165569(n)).

Extensions

a(16)-a(23) from Donovan Johnson, May 13 2010
a(24)-a(26) from Amiram Eldar, Nov 28 2019
Showing 1-4 of 4 results.

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