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%I #6 Apr 12 2017 23:17:19
%S 2,5,18,21,24,27,30,33,36,49,52,55,58,61,64,77,80,83,86,89,92,105,108,
%T 111,114,117,120,123,136,139,142,145,148,151,164,167,170,173,176,179,
%U 182,195,198,201,204,207,210,223,226,229,232,235,238,251,254,257,260
%N Powers of 10 that reach ...,7,8,4,2,1 under the mapping: if n is even divide by 2 else add 1.
%H G. C. Greubel, <a href="/A080689/b080689.txt">Table of n, a(n) for n = 1..1000</a>
%t a[n_] := MemberQ[Partition[NestWhileList[If[EvenQ[#], #/2, # + 1] &, 10^n, # != 1 &], 5, 1], {7, 8, 4, 2, 1}]; Select[Range[500], a] (* _G. C. Greubel_, Apr 12 2017 *)
%o (PARI) xnp10(n,p) = { for(x=1,n, p10 = 10^x; while(p10>1, if(p10%2==0,p10/=2,p10 = p10*p+1;); if(p10==7,print1(x" ")) ) ) }
%Y Cf. A022803.
%K easy,nonn
%O 1,1
%A _Cino Hilliard_, Mar 23 2003
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