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A124607
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Position of the first 7 in the decimal expansion of the square root of n, or -1 if 7 never appears.
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0
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-1, -1, 12, 2, -1, 7, 8, 5, 7, -1, 6, 8, 13, 9, 2, 3, -1, 13, 10, 15, 3, 6, 8, 2, 6, -1, 14, 11, 28, 10, 3, 4, 23, 2, 18, 6, -1, 5, 15, 7, 14, 10, 5, 4, 11, 2, 2, 24, 12, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, -1, 7, 47, 8, 26, 15, 14, 8, 9, 8, 10, 10, 2, 2, 5, 10, 6, -1, 13, 9, 21, 10, 3, 4, 28, 27, 16, 52, 24, 8, 8, 2, 2, 7, 31, 6, -1
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OFFSET
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0,3
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COMMENTS
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There is only one number -- 20,976 -- out of the first 100,000 integers for which the term in this sequence is greater than 100. Its term is 133. - Harvey P. Dale, Nov 17 2019
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LINKS
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MATHEMATICA
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Table[Position[RealDigits[Sqrt[n], 10, 200][[1]], 7, 1, 1]/.{}->-1, {n, 0, 200}]//Flatten (* Harvey P. Dale, Nov 17 2019 *)
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PROG
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(PARI) digitpos(n, m) = /* m-th digit in sqrt expansions */ { local(x, y, r, dot); default(realprecision, 1000); for(x=0, n, r = sqrt(x); if(issquare(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) = /* Revised 2007 */ { local(lnm, lns, tstr, vstr, x, j); vstr=Vec(Str(str)); match=Str(match); lns=length(str); lnm=length(match); for(x=1, lns-lnm+1, tstr=""; for(j=x, x+lnm-1, tstr=concat(tstr, vstr[j]); ); if(match==tstr, return(x)) ); return(0); }
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CROSSREFS
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KEYWORD
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base,easy,sign
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AUTHOR
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STATUS
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approved
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