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A082473 Numbers n such that n = phi(x)*core(x) for some x <= n, where phi(x) is the Euler totient function and core(x) the squarefree part of x. 7
1, 2, 6, 8, 12, 20, 32, 40, 42, 48, 54, 84, 108, 110, 120, 128, 156, 160, 192, 220, 240, 252, 272, 312, 336, 342, 432, 486, 500, 504, 506, 512, 544, 640, 660, 684, 768, 812, 840, 880, 930, 936, 960, 972, 1000, 1012, 1080, 1248, 1320, 1332, 1344, 1624, 1632 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also numbers n such that n = y*phi(y) for a unique positive integer y (see A194507). - Franz Vrabec, Aug 27 2011

Sequence A002618 sorted into ascending order; also A327171 sorted into ascending order, with duplicate terms removed.  Indices of nonzero terms in A327170 and in A327172. - Antti Karttunen, Sep 29 2019

REFERENCES

József Sándor and Borislav Crstici, Handbook of Number theory II, Kluwer Academic Publishers, 2004, Chapter 3, p. 224.

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1000 from Alois P. Heinz)

Walther Janous, Problem 6588, Advanced Problems, The American Mathematical Monthly, Vol. 95, No. 10 (1988), p. 963; How Often is n*phi(n) <= x^2?, Solution to Problem 6588, ibid., Vol. 98, No. 5 (1991), pp. 446-448.

FORMULA

From Antti Karttunen, Sep 29 2019: (Start)

a(n) = A002618(A194507(n)).

A327172(a(n)) = A194507(n).

(End)

The number of terms not exceeding x is ~ c * sqrt(x), where c = Product_{p prime} (1 + 1/sqrt(p*(p-1)) - 1/p) = 1.3651304521... (Janous, 1988). - Amiram Eldar, Mar 10 2021

MATHEMATICA

With[{nn = 1700}, TakeWhile[Union@ Array[EulerPhi[#] (Sqrt@ # /. (c_: 1) a_^(b_: 0) :> (c a^b)^2) &, nn], # <= nn &]] (* Michael De Vlieger, Sep 29 2019, after Bill Gosper at A007913 *)

PROG

(PARI) isok(n) = {for (x=1, n, if (eulerphi(x)*core(x) == n, return (1)); ); return (0); } \\ Michel Marcus, Dec 04 2013

CROSSREFS

Cf. A000010, A002618, A007913, A194507, A327170, A327171, A327172.

Sequence in context: A280236 A177869 A138639 * A325177 A263312 A226818

Adjacent sequences:  A082470 A082471 A082472 * A082474 A082475 A082476

KEYWORD

nonn

AUTHOR

Benoit Cloitre, Apr 27 2003

STATUS

approved

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Last modified June 19 15:58 EDT 2021. Contains 345144 sequences. (Running on oeis4.)