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A135242 Numbers n such that sigma(n)=2*(n-reversal(n)). 1
10, 51, 430, 591, 922, 5991, 8350, 20210, 28810, 59211, 59991, 6673581, 8179030, 20158910, 203887910, 833549650, 2263048910, 2814442310, 7823347543, 78551146723, 81896108650, 281444442310, 770920338133, 832428872110, 7670112536751 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

If n=6*10^m-9 is semiprime then n is in the sequence (the proof is easy).

a(26) > 10^13. - Giovanni Resta, Jul 13 2015

LINKS

Table of n, a(n) for n=1..25.

EXAMPLE

sigma(51)=72=2*(51-15)=2*(51-reversal(51)), so 51 is in the sequence.

MATHEMATICA

Do[If[2(n-FromDigits@Reverse@IntegerDigits@n)==DivisorSigma[1, n], Print[n]], {n, 70000000}]

CROSSREFS

Cf. A135243.

Sequence in context: A030183 A224327 A219573 * A041186 A058827 A232909

Adjacent sequences:  A135239 A135240 A135241 * A135243 A135244 A135245

KEYWORD

base,more,nonn

AUTHOR

Farideh Firoozbakht, Jan 04 2008

EXTENSIONS

a(15)-a(21) from Donovan Johnson, Dec 21 2008

a(22)-a(25) from Giovanni Resta, Jul 13 2015

STATUS

approved

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Last modified June 18 04:41 EDT 2021. Contains 345098 sequences. (Running on oeis4.)