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A070305
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Numbers n such that Card(k>0 : phi(k)=phi(n)) = tau(n).
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2
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2, 4, 8, 10, 11, 14, 16, 23, 27, 28, 29, 31, 32, 38, 47, 53, 59, 64, 67, 71, 79, 83, 86, 100, 103, 107, 114, 125, 127, 128, 131, 136, 137, 139, 147, 149, 151, 167, 170, 172, 173, 176, 179, 191, 197, 199, 202, 211, 223, 227, 229, 235, 239, 251, 256, 263, 265, 269
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OFFSET
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1,1
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LINKS
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FORMULA
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MATHEMATICA
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With[{nn = 300}, Function[s, DeleteCases[MapIndexed[If[DivisorSigma[0, First@ #2] == #1, First@ #2, 0] &, Take[#, nn]], 0] &@ Values@ KeySort@ Flatten@ Map[Function[{k, m}, Map[# -> m &, k]] @@ {#, Length@ #} &@ Lookup[s, #] &, Keys@ s]]@ KeySort@ PositionIndex@ Array[EulerPhi, Floor[nn^(4/3)] + 10]] (* Michael De Vlieger, Jul 18 2017 *)
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PROG
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(PARI) for(n=1, 350, if(sum(i=1, 10*n, if(eulerphi(n)-eulerphi(i), 0, 1))==numdiv(n), print1(n, ", "))) \\ By the original author. Note: the upper limit 10*n for the search range is quite ad hoc, and is guaranteed to miss some cases when n is large enough. Cf. Wikipedia-article. - Antti Karttunen, Jul 19 2017
(PARI)
;; Here is an implementation not using arbitrary limits:
A014197(n, m=1) = { n==1 && return(1+(m<2)); my(p, q); sumdiv(n, d, if( d>=m && isprime(d+1), sum( i=0, valuation(q=n\d, p=d+1), A014197(q\p^i, p))))} \\ M. F. Hasler, Oct 05 2009
isA070305(n) = (A066412(n) == numdiv(n));
n=0; k=1; while(k <= 1000, n=n+1; if(isA070305(n), write("b070305.txt", k, " ", n); k=k+1)); \\ Antti Karttunen, Jul 19 2017
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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