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A147549
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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.
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2
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0, 0, 3, 1, 3, 4, 11, 17, 116, 25, 222, 1806, 54, 223
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,3
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COMMENTS
| 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.
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MATHEMATICA
| 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}]
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CROSSREFS
| Cf. A147547, A147548.
Sequence in context: A008924 A175186 A021323 * A177355 A076157 A087493
Adjacent sequences: A147546 A147547 A147548 * A147550 A147551 A147552
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KEYWORD
| hard,more,nonn,base
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AUTHOR
| Farideh Firoozbakht (mymontain(AT)yahoo.com), Nov 12 2008
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EXTENSIONS
| a(10)..a(14) from Max Alekseyev (maxale(AT)gmail.com), Mar 12 2009
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