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A136544 Numbers n such that phi(n)+sigma(n)=reversal(n)+3. 4
3, 3997, 3999997, 168632373, 399999999997, 3999999999997 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Next term of the sequence is greater than 10^9. All semiprimes of the form 4*10^m-3 are in the sequence - the proof is easy. For m=3,6,11,12,13,15,16,18,19,24,38,56,60,... 4*10^m-3 is semiprime. Is it true that 3 is the only prime term in the sequence?
a(7) > 10^13. - Giovanni Resta, Feb 08 2014
LINKS
EXAMPLE
phi(168632373)+sigma(168632373)=87744384+285492480=373236861+3= reversal(168632373)+3, so 168632373 is in the sequence.
MATHEMATICA
Do[If[DivisorSigma[1, n]+EulerPhi@n==FromDigits@Reverse@IntegerDigits@n+ 3, Print[n]], {n, 500000000}]
CROSSREFS
Sequence in context: A196628 A089895 A116213 * A024048 A094319 A362536
KEYWORD
base,more,nonn
AUTHOR
Farideh Firoozbakht, Jan 20 2008
EXTENSIONS
a(5) from Giovanni Resta, Nov 03 2012
a(6) from Giovanni Resta, Feb 06 2014
STATUS
approved

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Last modified June 21 04:30 EDT 2024. Contains 373540 sequences. (Running on oeis4.)