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Search: id:A147549
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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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+0
3
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| 0, 0, 3, 1, 3, 4, 11, 17, 116, 25, 222, 1806, 54, 223
(list; graph; listen)
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OFFSET
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1,3
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COMMENT
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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
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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
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Cf. A147547, A147548.
Sequence in context: A162932 A008924 A021323 this_sequence A076157 A087493 A118125
Adjacent sequences: A147546 A147547 A147548 this_sequence A147550 A147551 A147552
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KEYWORD
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hard,more,nonn,base
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AUTHOR
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Farideh Firoozbakht (mymontain(AT)yahoo.com), Nov 12 2008
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EXTENSIONS
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a(10)..a(14) from Max Alekseyev (maxale(AT)gmail.com), Mar 12 2009
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