Search: id:A147549
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%I A147549
%S A147549 0,0,3,1,3,4,11,17,116,25,222,1806,54,223
%N A147549 a(n) is the number of n-digit numbers m such that phi(m)=phi(10^n+1), 
               gcd(10^n+1,m)=1 & 10 doesn't divide m.
%C A147549 If 10^n+1 is prime (n must be of the form 2^k) then a(n)=0 because in 
               this case there is no n-digit number m such that phi(10^n+1)=10^n=phi(m). 
               For answering to a question of Maximilian Hasler (Nov 06, 2008) about 
               infinteness of the "primitive" elements (those which aren't a multiple 
               of 10) of the sequence A147619 I defined this sequence and the sequences 
               A147547 & A147548.
%t A147549 a[n_]:=(b=10^n+1;c=EulerPhi[b];e=b-2; If[PrimeQ[b],0,Length[Select[Range[ 
               c+1,e],Mod[ #,10]>0 && GCD[ #,b]==1 && EulerPhi[b]==EulerPhi[ # ]&]]]); 
               Do[Print[a[n]],{n,9}]
%Y A147549 Cf. A147547, A147548.
%Y A147549 Sequence in context: A162932 A008924 A021323 this_sequence A076157 A087493 
               A118125
%Y A147549 Adjacent sequences: A147546 A147547 A147548 this_sequence A147550 A147551 
               A147552
%K A147549 hard,more,nonn,base
%O A147549 1,3
%A A147549 Farideh Firoozbakht (mymontain(AT)yahoo.com), Nov 12 2008
%E A147549 a(10)..a(14) from Max Alekseyev (maxale(AT)gmail.com), Mar 12 2009

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