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A230004 Numbers n such that phi(n) + sigma(n) = reversal(n) + 4. 6
499, 2836, 4999, 49999, 280036, 4999999, 28000036, 283682836, 2800000000036 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

If p=5*10^m-1 is prime (m is a term of A056712) then p is in the sequence.

Let p(m,n) = 10^(m+3)*(7*10^(m+2)+92)*(10^((m+4)*n)-1)/(10^(m+4)-1) +7*10^(m+1)+9, if m>0, n>=0 and p(m,n) is prime then 4*p(m,n) is in the sequence.

All known terms are of these two forms.

What is the smallest term of the sequence which is not of the form p or 4*p where p is prime?

Note that a(2)=4*p(1,0), a(5)=4*p(3,0), a(7)=4*p(5,0) and a(8)=4*p(1,1).

LINKS

Table of n, a(n) for n=1..9.

EXAMPLE

phi(499)+sigma(499) = 498+500 = 994+4 = reversal(499)+4, so 499 is in the sequence.

MATHEMATICA

r[n_] := FromDigits[Reverse[IntegerDigits[n]]]; Do[If[DivisorSigma[1, n] + EulerPhi[n] == r[n] + 4, Print[n]], {n, 1050000000}]

PROG

(PARI) is(n)=subst(Polrev(digits(n)), 'x, 10)+4==eulerphi(n)+sigma(n) \\ Charles R Greathouse IV, Nov 08 2013

CROSSREFS

Cf. A000010, A000203, A004086, A056712, A230005.

Sequence in context: A243957 A259890 A178044 * A203735 A093945 A184146

Adjacent sequences:  A230001 A230002 A230003 * A230005 A230006 A230007

KEYWORD

nonn,base

AUTHOR

Farideh Firoozbakht, Nov 07 2013

EXTENSIONS

a(9) from Giovanni Resta, Feb 06 2014

STATUS

approved

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Last modified December 7 13:08 EST 2021. Contains 349581 sequences. (Running on oeis4.)