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A130913 Composite solutions to the equation phi(x)+sigma(x) = 2*reversal(x). 1
44722206, 69067317, 460647525, 485700768, 427528099084 (list; graph; refs; listen; history; text; internal format)
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
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
Sequence in context: A172718 A270611 A049360 * A213353 A205263 A087534
KEYWORD
hard,more,nonn,base
AUTHOR
Farideh Firoozbakht, Jun 09 2007
EXTENSIONS
a(5) from Giovanni Resta, Nov 03 2012
STATUS
approved

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Last modified March 18 22:24 EDT 2024. Contains 370951 sequences. (Running on oeis4.)