A124687 Position of the first 2 in the decimal expansion of the cube root of n, or -1 if this digit never appears.

-1, -1, 2, 4, 29, 28, 6, 4, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, -1, 16, 4, 5, 10, 7, 2, 2, 2, 6, 4, 50, 5, 23, 5, 6, 15, 12, 16, 35, 6, 5, 12, 24, 6, 4, 5, 27, 4, 3, 11, 12, 4, 21, 13, 33, 8, -1, 3, 5, 22, 29, 9, 3, 12, 17, 14, 18, 2, 2, 2, 2, 2, 50, 3, 21, 4, 17, 6, 10, 9, 19, 32, 25, 11, 25, 16, 24, 4, 11, 10, 8, 3, 13

Offset

0,3

Links

Chai Wah Wu, Table of n, a(n) for n = 0..10000

Prog

(PARI) digitposcbrt(n, m) = \ cbrt expansion { local(x, y, r, dot); for(x=0, n, r = (x^(1/3)); if(iscube(x), y=find(Str(floor(r)), m), y=find(Str(r), m); dot=find(Str(r), "."); if(dot < y, y--); ); if(y, print1(y", "), print1(-1", ") ) ) } find(str, match) = \Return the position of the first occurrence of string \match in string str { local(lnm, lns, x, c, i); str=Str(str); \This allows leaving quotes off input match=Str(match); c=0; i=0; lns=length(str); lnm=length(match); if(lnm>1, i=1); x=1; while(x<=lns-lnm+1, if(mid(str, x, lnm)== match, break, x++); ); if(x>lns, return(0), return(x)) } mid(str, s, n) = \ Get a substring of length n from string str starting at position s in str. { local(v, ln, x, tmp); v =""; tmp = Vec(str); ln=length(tmp); for(x=s, s+n-1, v=concat(v, tmp[x]); ); return(v) } iscube(n) = { local(r); r = n^(1/3); if(floor(r+.5)^3== n, 1, 0) }

Crossrefs

Sequence in context: A329102 A126580 A329063 * A267122 A018291 A033167

Adjacent sequences: A124684 A124685 A124686 * A124688 A124689 A124690

Keyword

base,easy,sign

Author

Cino Hilliard, Dec 25 2006

Status

approved