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A136538
Numbers n such that reversal(n)=2*phi(n).
2
2, 4, 8, 42, 84, 2763, 4032, 8064, 67314, 86558, 291483, 2700063, 2700000063, 4039603962, 46420566582, 6739054689866
OFFSET
1,1
COMMENTS
If m>1 and p=3*10^m+7 is prime then n=9*p is in the sequence (the proof is easy). If n is an even term of the sequence and the largest digit of n is less than 5(3) then 2n is (both numbers 2n & 4n are) in the sequence (the proof is easy).
a(17) > 10^13. - Giovanni Resta, Aug 12 2019
EXAMPLE
Reversal(42)=24=2*12=2*phi(42), so 42 is in the sequence. [Example corrected Jan 25 2008]
MATHEMATICA
Do[If[FromDigits@Reverse@IntegerDigits@n==2*EulerPhi[n], Print[n]], {n, 100000000}]
CROSSREFS
KEYWORD
nonn,base,more
AUTHOR
Farideh Firoozbakht, Jan 04 2008
EXTENSIONS
a(13)-a(15) from Giovanni Resta, Oct 28 2012
a(16) from Giovanni Resta, Aug 12 2019
STATUS
approved