A044631 - OEIS

%I #28 Jul 15 2014 12:05:04

%S 63,127,191,255,319,383,447,511,575,639,703,767,831,895,959,1023,1087,

%T 1151,1215,1279,1343,1407,1471,1535,1599,1663,1727,1791,1855,1919,

%U 1983,2047,2111,2175,2239,2303,2367,2431,2495,2559,2623,2687,2751,2815,2879,2943

%N Numbers n such that string 7,7 occurs in the base 8 representation of n but not of n+1.

%H Vincenzo Librandi, <a href="/A044631/b044631.txt">Table of n, a(n) for n = 1..1000</a>

%H Tanya Khovanova, <a href="http://www.tanyakhovanova.com/RecursiveSequences/NonRecursions.html">Non Recursions</a>

%t f[n_] := Length[StringPosition[ToString[FromDigits[IntegerDigits[n, 8]]], "77", 1]]; a1 = Select[Table[n, {n, 10000}], f[#] > 0 && f[# + 1] == 0 &] (* _Vladimir Joseph Stephan Orlovsky_, Jul 14 2011 *)

%o (PARI) has77(n)=my(t);while(n,if(bitand(n,7)==7,if(t,return(1),t=1),t=0);n>>=3);0

%o is(n)=has77(n)&&!has77(n+1) \\ _Charles R Greathouse IV_, Jul 14 2011

%K nonn,base

%O 1,1

%A _Clark Kimberling_