|
| |
|
|
A130913
|
|
Composite solutions to the equation phi(x)+sigma(x) = 2*reversal(x).
|
|
1
|
| |
|
|
|
OFFSET
|
1,1
|
|
|
COMMENTS
|
For a prime p we have phi(p)+sigma(p) = 2*reversal(p) iff p is a palindrome.
a(6) > 10^12. - Giovanni Resta, Nov 03 2012
|
|
|
LINKS
|
Table of n, a(n) for n=1..5.
|
|
|
EXAMPLE
|
485700768 is in the sequence because 485700768 is composite and phi(485700768)+sigma(485700768) = 1734015168 = 2*867007584.
|
|
|
MATHEMATICA
|
Do[a=FromDigits[Reverse[IntegerDigits[n]]]; If[a>n&&EulerPhi[n]+ DivisorSigma[1, n]==2a, Print[n]], {n, 250000000}] (* Farideh Firoozbakht, Nov 26 2013 *)
|
|
|
CROSSREFS
|
Cf. A000010, A000203, A004086, A130000, A230020.
Sequence in context: A172718 A270611 A049360 * A213353 A205263 A087534
Adjacent sequences: A130910 A130911 A130912 * A130914 A130915 A130916
|
|
|
KEYWORD
|
hard,more,nonn,base
|
|
|
AUTHOR
|
Farideh Firoozbakht, Jun 09 2007
|
|
|
EXTENSIONS
|
a(5) from Giovanni Resta, Nov 03 2012
|
|
|
STATUS
|
approved
|
| |
|
|
