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A114928 Numbers n such that sigma(n)=4*reversal(n). 3
42, 402, 492, 4000002, 57906504, 400000002, 4000000002, 6279090751, 62698513951, 400000000002 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

If p=(2*10^n+1)/3 is prime then m=6*p is in the sequence because sigma(m)=sigma(6*p)=12*(2*10^n+4)/3=4*(2*10^n+4)=4* reversal(4*10^n+2)=4*reversal(6*(2*10^n+1)/3)=4*reversal(6*p) =4*reversal(m). Next term is greater than 5*10^8.

a(11) > 10^12. - Giovanni Resta, Oct 28 2012

LINKS

Table of n, a(n) for n=1..10.

EXAMPLE

492 is in the sequence because sigma(492)=sigma(4*3*41)=7*4*42

=4*294=4*reversal(492).

MATHEMATICA

Do[If[DivisorSigma[1, n]==4*FromDigits[Reverse[IntegerDigits[n]]], Print[n]], {n, 500000000}]

CROSSREFS

Cf. A069216, A105324, A114927.

Sequence in context: A064302 A231705 A224455 * A127545 A027318 A251425

Adjacent sequences:  A114925 A114926 A114927 * A114929 A114930 A114931

KEYWORD

base,more,nonn

AUTHOR

Farideh Firoozbakht, Jan 28 2006

EXTENSIONS

a(7)-a(9) from Donovan Johnson, Dec 21 2008

a(10) from Giovanni Resta, Oct 28 2012

STATUS

approved

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Last modified August 13 08:18 EDT 2022. Contains 356079 sequences. (Running on oeis4.)