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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 October 15 00:14 EDT 2019. Contains 328025 sequences. (Running on oeis4.)