OFFSET
1,1
COMMENTS
If p=10^m-3 is prime then 30*p is in the sequence because phi(30*p)=phi(30)*phi(p)=8*(10^m-4)=4*(2*10^m-8)=4*reversal (3*10^m-9)=4*reversal(3*p)=4*reversal(30*p). Next term is greater than 55*10^7.
Let f(m,n)=(78*10^(m+3)+210)*(10^(n*(m+4))-1)/(10^(m+4)-1)+7, if p=f(m,n) is prime then 30*p is a term of the sequence. - Jahangeer Kholdi, Nov 13 2013
Also if p=(1/101)*(680*10000^n+27) is prime then 60*p is in the sequence. - Jahangeer Kholdi, Nov 13 2013
a(30) > 10^13. - Giovanni Resta, Aug 12 2019
EXAMPLE
20 is in the sequence because phi(20)=4*2=4*reversal(20).
MATHEMATICA
Do[If[EulerPhi[n]==4*FromDigits[Reverse[IntegerDigits[n]]], Print[n]], {n, 550000000}]
CROSSREFS
KEYWORD
nonn,base,more
AUTHOR
Farideh Firoozbakht, Jan 29 2006
EXTENSIONS
a(21)-a(27) from Giovanni Resta, Oct 28 2012
a(28)-a(29) from Giovanni Resta, Aug 12 2019
STATUS
approved