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 A327172 If there is a divisor d of n such that phi(d)*d = n, then a(n) = d, otherwise a(n) = 0. 5
 1, 2, 0, 0, 0, 3, 0, 4, 0, 0, 0, 6, 0, 0, 0, 0, 0, 0, 0, 5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 8, 0, 0, 0, 0, 0, 0, 0, 10, 0, 7, 0, 0, 0, 0, 0, 12, 0, 0, 0, 0, 0, 9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 14, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 18, 0, 11, 0, 0, 0, 0, 0, 0, 0, 0, 0, 15 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS If such a divisor exists, it is necessarily unique. See Franz Vrabec's Dec 12 2012 comment in A002618. Each natural number n > 0 occurs exactly once in this sequence, at position A002618(n). LINKS Antti Karttunen, Table of n, a(n) for n = 1..21000 Antti Karttunen, Data supplement: n, a(n) computed for n = 1..100000 FORMULA a(A002618(n)) = n. a(A082473(n)) = A194507(n). MATHEMATICA With[{s = EulerPhi /@ Range@ 120}, Table[DivisorSum[n, # &, # s[[#]] == n &], {n, Length@ s}]] (* Michael De Vlieger, Sep 29 2019 *) PROG (PARI) A327172(n) = { fordiv(n, d, if(eulerphi(d)*d == n, return(d))); (0); }; CROSSREFS Left inverse of A002618. Cf. A000010. Cf. A082473 (the indices of nonzero terms), A194507 (nonzero terms in the order of appearance). Sequence in context: A292374 A292376 A257685 * A113503 A082507 A132349 Adjacent sequences:  A327169 A327170 A327171 * A327173 A327174 A327175 KEYWORD nonn AUTHOR Antti Karttunen, Sep 28 2019 STATUS approved

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Last modified May 16 08:07 EDT 2021. Contains 343940 sequences. (Running on oeis4.)