login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A340058 Composite numbers c such that phi(c)/phi(mind(c)) mod phi(c)/phi(maxd(c)) = 0, where phi is the Euler function, mind(c) is the smallest nontrivial divisor of c, maxd(c) is the largest nontrivial divisor of c. 3
4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 25, 26, 27, 28, 30, 32, 33, 34, 36, 38, 39, 40, 42, 44, 45, 46, 48, 49, 50, 51, 52, 54, 56, 57, 58, 60, 62, 63, 64, 65, 66, 68, 69, 70, 72, 74, 75, 76, 78, 80, 81, 82, 84, 85, 86, 87, 88, 90, 91, 92, 93, 94, 96, 98, 99 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

This equivalence criterion splits a set of composite numbers into two classes and can be used to count certain combinatorial objects.

LINKS

Table of n, a(n) for n=1..69.

PROG

(MATLAB)

n=100; % gives all terms of the sequence not exceeding n

A=[];

for i=1:n

   dn=divisors(i);

   if size(dn, 2)>2 && mod(totient(i)/totient(dn(2)), totient(i)/totient(dn(end-1)))==0

      A=[A i];

   end

end

function [res] = totient(n)

res=0;

    for i=1:n

        if gcd(i, n)==1

            res=res+1;

        end

    end

end

(PARI) isok(c) = if ((c>1) && !isprime(c), my(t=eulerphi(c), d=divisors(c)); ((t/eulerphi(d[2])) % (t/eulerphi(d[#d-1]))) == 0); \\ Michel Marcus, Dec 28 2020

CROSSREFS

Cf. A000010, A002808, A335902.

Sequence in context: A140209 A077091 A340268 * A174891 A051035 A046349

Adjacent sequences:  A340055 A340056 A340057 * A340059 A340060 A340061

KEYWORD

nonn

AUTHOR

Maxim Karimov, Dec 27 2020

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified October 21 02:29 EDT 2021. Contains 348141 sequences. (Running on oeis4.)