OFFSET
1,4
COMMENTS
A variant of Conway's 'Look-and-Say' sequence A005150, without run cut-off. It describes at each step the preceding digits taken altogether.
There are different optional rules to build such a sequence. This method 2 does not consider already said digits.
As in the original Look-and-Say sequence, a(n) is always equal to 1, 2 or 3. The subsequence 3,3,3 never appears.
The sequence is determined by pairs of digits. Terms of even rank are counts while terms of odd rank are figures.
LINKS
J.-C. Hervé, Table of n, a(n) for n = 1..10000
EXAMPLE
a(1) = 1, then a(2) = 1 and a(3) = 1 (one 1). Leaving out the first 1 already said, we now have two 1's, then a(4) = 2 and a(5) = 1, and then a(6) = 1, a(7) = 2, a(8) = 2, a(9) = 1, etc.
MATHEMATICA
n = 100; a[0] = 1; see = say = 0; While[say < n - 1, c = 0; dg = a[see]; While[see <= say, If[a[see] == dg, c += 1, Break[]]; see += 1]; a[++say] = c; If[say < n - 1, a[++say] = dg]]; Array[a, n, 0] (* Jean-François Alcover, Jul 11 2013, translated and adapted from J.-C. Hervé's C program *)
PROG
(C) /* computes first n terms in array a[] */
int *swys(int n) {
int a[n] ;
int see, say, c ;
a[0] = 1;
see = say = 0 ;
while( say < n-1 ) {
c = 0 ; /* count */
dg = a[see] /* digit */
while (see <= say) {
if (a[see]== dg) c += 1 ;
else break ;
see += 1 ;
}
a[++say] = c ;
if (say < n-1) a[++say] = dg ;
}
return(a);
}
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Jean-Christophe Hervé, May 05 2013
STATUS
approved