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A136543 Numbers n such that phi(n)+sigma(n)=4*reversal(n). 2
793, 79993, 2152311, 79999993, 7999999993, 799999999993 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

All semiprimes of the form 8*10^m-7 are in the sequence - the proof is easy. Next term is greater than 10^8.

a(7) > 10^12. - Giovanni Resta, Nov 03 2012

A100412(n) is in the sequence for n = 2, 4, 7, 9, 11, 16, 18, 23, 31, 32, 40,... - M. F. Hasler, Nov 03 2012

LINKS

Table of n, a(n) for n=1..6.

EXAMPLE

phi(2152311)+sigma(2152311)=1217664+3312384=4*1132512=4*reversal(2152311), so 2152311 is in the sequence.

MATHEMATICA

Do[If[4*FromDigits@Reverse@IntegerDigits@n==EulerPhi@n+ DivisorSigma[1, n], Print[n]], {n, 100000000}]

CROSSREFS

Sequence in context: A133537 A213471 A075667 * A133274 A261657 A086393

Adjacent sequences:  A136540 A136541 A136542 * A136544 A136545 A136546

KEYWORD

base,more,nonn

AUTHOR

Farideh Firoozbakht, Jan 20 2008

EXTENSIONS

a(5)-a(6) from Giovanni Resta, Nov 03 2012

STATUS

approved

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Last modified June 24 07:19 EDT 2017. Contains 288697 sequences.