1,1

Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1000 from Antti Karttunen)

Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems (invphi.gp).

Wikipedia, Euler's totient function (see the last paragraph in section "Some values of the function").

Numbers k such that A066412(k) = A000005(k).

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 *)

(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

(PARI)

A066412(n) = A014197(eulerphi(n));

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

(PARI) is(m) = {my(f = factor(m)); invphiNum(eulerphi(f)) == numdiv(f); } \\ Amiram Eldar, Nov 19 2024, using Max Alekseyev's invphi.gp

(Scheme)

;; With my IntSeq-library.

(define A070305 (MATCHING-POS 1 1 (lambda (n) (= (A066412 n) (A000005 n))))) ;; Antti Karttunen, Jul 18 2017

Cf. A000005, A000010, A057826, A066412.

Sequence in context: A028838 A178332 A265155 * A174567 A178330 A029992

Adjacent sequences: A070302 A070303 A070304 * A070306 A070307 A070308

easy,nonn

Benoit Cloitre, May 10 2002

approved