login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A230005 Numbers n such that phi(n) + sigma(n) = reversal(n) - 4. 6
489, 4629, 296206, 460029, 29589106, 46000029, 2927272726, 4045046518, 21223345084, 29600331295, 296151515206, 460000000029 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

If p = 15*10^k+10+(10^k-1)/3 is prime then 3*p is in the sequence.

a(7) is greater than 1.3*10^8.

a(11) > 10^11. - Donovan Johnson, Nov 08 2013

If p=(1/11)*(23*100^m-1) is prime then 14*p is a term of the sequence. - Farideh Firoozbakht, Nov 08 2013

a(13) > 10^13. - Giovanni Resta, Feb 08 2014

If p = (1685*10^(2k+2)+31)/33 is prime then 58*p is in the sequence. For k = 0, 3, 9, 30, 42, 51, 120, 846, ... p is prime. - Farideh Firoozbakht, Feb 10 2014

LINKS

Table of n, a(n) for n=1..12.

EXAMPLE

489 is in the sequence because phi(489)+sigma(489) = 324+656 = 984-4 = reversal(489)-4.

MATHEMATICA

Do[If[FromDigits@Reverse@IntegerDigits@n-4 == EulerPhi[n] + DivisorSigma[1, n], Print[n]], {n, 130000000}]

PROG

(PARI) is(n)=subst(Polrev(digits(n)), 'x, 10)-4==eulerphi(n)+sigma(n) \\ Charles R Greathouse IV, Nov 08 2013

CROSSREFS

Cf. A000010, A000203, A004086, A070272, A230004, A230006, A230019, A136544, A237521, A237522.

Sequence in context: A201215 A214807 A251676 * A221940 A221739 A068751

Adjacent sequences:  A230002 A230003 A230004 * A230006 A230007 A230008

KEYWORD

nonn,base,more

AUTHOR

Farideh Firoozbakht, Nov 07 2013

EXTENSIONS

a(7)-a(10) from Donovan Johnson, Nov 08 2013

a(11)-a(12) from Giovanni Resta, Feb 06 2014

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 March 23 05:36 EDT 2019. Contains 321422 sequences. (Running on oeis4.)