

A125626


Numbers n whose reverse binary representation has the following property: let a 0 mean "halving" and a 1 mean "k > 3k+1". The number describes an operation k > f_n(k). If the equation f_n(k) = k has a positive solution, n is a term in the sequence.


5



4, 8, 16, 32, 33, 34, 36, 40, 48, 64, 65, 66, 68, 72, 80, 96, 128, 129, 130, 131, 132, 133, 134, 136, 137, 138, 140, 144, 145, 146, 148, 152, 160, 161, 162, 164, 168, 176, 192, 193, 194, 196, 200, 208, 224, 256, 257, 258, 259, 260
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OFFSET

4,1


COMMENTS

The terms in this sequence have the following characterization. Suppose the binary expansion of n contains i 1's and j 0's. Then it is easy to see that n is in the sequence if and only if 3^i < 2^j, or i/j < log 2 / log 3 = .630929753...  David Applegate and N. J. A. Sloane, Feb 01 2007.
Note that f_n(x) is always a linear function of x.
The reverse binary expansions of the first few terms are:
001
0001
00001
000001
100001
010001
001001
000101
000011
0000001
1000001
0100001
0010001
0001001
0000101
0000011
00000001
10000001
01000001
11000001
00100001
...
Could be used in conjunction with the Collatz (or 3x+1) conjecture. If the positive solution k is an integer (most are not) then a cycle exists. If this cycle does not contain a 1 and the sequence of steps agrees with what Collatz's rule tells you to do when you start with k, then the Collatz conjecture would be false.


LINKS

Table of n, a(n) for n=4..53.


EXAMPLE

Consider the term 200: its binary representation is 11001000. Reversing this gives 00010011. We solve (3*(3*(((3*(((k/2)/2)/2)+1)/2)/2)+1)+1) = k and find k = 40. Since k is positive, 200 is a member of the sequence.


PROG

(C) #include <stdio.h> #include <stdlib.h> #include <math.h> void multiply(float *coef, float *cons) { (*coef) *= 3; (*cons) = 3*(*cons)+1; } void divide(float *coef, float *cons) { (*coef) /= 2; (*cons) /= 2; } int main() { int a, b, c, n; float coef, cons, final; char data[30], sequence[30]; for (a = 1; a < 500; a++) { coef = 1; cons = 0; c = a; sequence[0] = ''; for (b = 1; b < 12; b++) //12 is arbitrary; it allows for "a" up to 2^12 { if (c != 0) { if (c % 2) { sprintf(sequence, "%s1", sequence); multiply(&coef, &cons); } else { sprintf(sequence, "%s0", sequence); divide(&coef, &cons); } c = trunc(c/2); } else break; } if (coef >= 1.0) { coef = 1.0; cons *= 1.0; } else coef = 1.0coef; final = cons/coef; if (final > 0) { sprintf(data, "%10.3f %s %d ", final, sequence, a); printf(data); } } return 0; }


CROSSREFS

For the values of n for which the fixed point k is a positive (or any) integer, see A125754A125757.
Cf. A112695, A125710, A125711.
Sequence in context: A215349 A215348 A053163 * A141031 A061011 A181800
Adjacent sequences: A125623 A125624 A125625 * A125627 A125628 A125629


KEYWORD

easy,nonn


AUTHOR

Nicholas Sanders (gummybean(AT)gmail.com), Jan 27 2007


STATUS

approved



