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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

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

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Last modified May 19 18:35 EDT 2022. Contains 353847 sequences. (Running on oeis4.)