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A156793
Indices of primes whose reversal is a different prime.
2
6, 7, 11, 12, 20, 21, 22, 25, 28, 30, 35, 37, 39, 41, 46, 64, 68, 69, 72, 77, 126, 127, 130, 131, 132, 133, 135, 136, 155, 159, 160, 162, 163, 164, 166, 167, 169, 172, 173, 174, 178, 180, 182, 184, 185, 186, 190, 191, 194, 196, 197, 198, 199, 200, 201, 202, 203
OFFSET
1,1
COMMENTS
Indices of primes in A006567.
LINKS
EXAMPLE
If prime(n=6)=13(1<3) and 31=prime, then 6=a(1). If prime(n=7)=17(1<7) and 71=prime, then 7=a(2). If prime(n=11)=31(3>1) and 13=prime, then 11=a(3), etc.
MATHEMATICA
difpQ[n_]:=Module[{idn=IntegerDigits[n], ridn}, ridn=Reverse[idn]; idn!=ridn && PrimeQ[FromDigits[ridn]]]; PrimePi/@ Select[ Prime[ Range[ 250]], difpQ] (* Harvey P. Dale, Nov 23 2011 *)
CROSSREFS
Cf. A006567.
Sequence in context: A015825 A243489 A164018 * A081715 A054504 A190612
KEYWORD
nonn,base
AUTHOR
EXTENSIONS
45 replaced by 46. 127, 131, 136, 155, 159 inserted, etc. - R. J. Mathar, Feb 26 2009
STATUS
approved