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A213893 Fixed points of a sequence h(n) defined by the minimum number of 4's in the relation n*[n,4,4,...,4,n] = [x,...,x] between simple continued fractions. 3
3, 7, 43, 67, 103, 127, 163, 223, 283, 367, 463, 487, 523, 547, 607, 643, 727, 787, 823, 883, 907, 1063, 1123, 1303, 1327, 1423, 1447, 1543, 1567, 1627, 1663, 1723, 1747, 1783, 1867, 1987, 2083, 2143, 2203, 2287, 2347, 2383, 2467, 2683, 2707, 2767, 2803, 2887 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

In a variant of A213891, multiply n by a number with simple continued fraction [n,4,4,...,4,n] and increase the number of 4's until the continued fraction of the product has the same first and last entry (called x in the NAME). Examples are

2*[2,4,2] = [4,2,4],

3*[3,4,4,4,3] = [9,1,2,2,2,1,9],

4*[4,4,4] = [16,1,16],

5*[5,4,4,4,4,5] = [26,5,1,1,5,26].

The number of 4's needed defines the sequence h(n) = 1, 3, 1, 4, 3, 7, 3, 3, 9, ... (n>=2).

The current sequence contains the fixed points of h, i.e., those n where h(n)=n.

We conjecture that this sequence contains prime numbers analogous to the sequence of prime numbers A000057, in the sense that, instead of referring to the Fibonacci sequences(sequences satisfying f(n) = f(n-1) + f(n-2) with arbitrary positive integer values for f(1) and f(2)) it refers to the sequences satisfying f(n) = 4*f(n-1) + f(n-2), A001076, A001077, A015448, etc. This would mean that a prime is in the sequence if and only if it divides some term in each of the sequences satisfying f(n) = 4*f(n-1) + f(n-2).

The above sequence h() is recorded as A262214. - M. F. Hasler, Sep 15 2015

LINKS

Table of n, a(n) for n=1..48.

MATHEMATICA

f[m_, n_] := Block[{c, k = 1}, c[x_, y_] := ContinuedFraction[x FromContinuedFraction[Join[{x}, Table[m, {y}], {x}]]]; While[First@ c[n, k] != Last@ c[n, k], k++]; k]; Select[Range[2, 1000], f[4, #] == # &] (* Michael De Vlieger, Sep 16 2015 *)

PROG

(PARI)

{a(n) = local(t, m=1); if( n<2, 0, while( 1,

   t = contfracpnqn( concat([n, vector(m, i, 4), n]));

   t = contfrac(n*t[1, 1]/t[2, 1]);

   if(t[1]<n^2 || t[#t]<n^2, m++, break));

m)};

for(k=1, 3000, if(k==a(k), print1(a(k), ", ")));

CROSSREFS

Cf. A000057, A213891, A213892, A213894 - A213899, A261311; A213358.

Cf. A213648, A262212 - A262220, A213900, A262211.

Sequence in context: A337829 A257366 A141304 * A236476 A282178 A101208

Adjacent sequences:  A213890 A213891 A213892 * A213894 A213895 A213896

KEYWORD

nonn

AUTHOR

Art DuPre, Jun 23 2012

STATUS

approved

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Last modified September 21 05:02 EDT 2021. Contains 347596 sequences. (Running on oeis4.)