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 A230019 Numbers n such that sigma(n) + phi(n) = reversal(n) - 3. 4
 199993, 17333334, 19999993, 199999999993, 1999999999993 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS If n=2*10^m-7 is a semiprime then n is in the sequence. Also if p=(1/999)*(962*1000^m+37) is prime then 18*p is in the sequence. All known terms are of these two forms. a(6) > 10^13. - Giovanni Resta, Feb 08 2014 LINKS Table of n, a(n) for n=1..5. MATHEMATICA r[n_] := FromDigits[Reverse[IntegerDigits[n]]]; Do[If[DivisorSigma[1, n] + EulerPhi[n] == r[n]-3, Print[n]], {n, 1000000000}] CROSSREFS Cf. A000010, A000203, A004086, A070272, A230004, A230005, A230006, A136544, A237521, A237522. Sequence in context: A094799 A163500 A164321 * A106777 A094800 A200863 Adjacent sequences: A230016 A230017 A230018 * A230020 A230021 A230022 KEYWORD nonn,base,more AUTHOR Farideh Firoozbakht, Dec 02 2013 EXTENSIONS a(4)-a(5) from Giovanni Resta, Feb 06 2014 STATUS approved

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Last modified September 10 17:45 EDT 2024. Contains 375792 sequences. (Running on oeis4.)