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A069469
Numbers n such that prime(reversal(n)) = reversal(prime(n)). Ignore leading 0's.
2
1, 2, 3, 4, 5, 12, 21, 8114118, 535252535
OFFSET
1,2
COMMENTS
For an arithmetical function f, call the arguments n such that f(reverse(n)) = reverse(f(n)) the "palinpoints" of f. This sequence is the sequence of palinpoints of f(n) = prime(n).
These are all the palinpoints of prime(n) not exceeding 10^7. There are more (535252535 is known to be a term, but it is not known whether it is the next one).
Contains all n such that n and prime(n) are both palindromes, i.e. A046942. Heuristically, we would expect there to be infinitely many of these, but they will be rare: the number of them with at most d digits may be on the order of sqrt(d). - Robert Israel, May 30 2016
a(10) > 10^9. - Giovanni Resta, Apr 13 2017
EXAMPLE
Let f(n) = Prime(n). Then f(21) = 73, f(12) = 37, so f(reverse(21)) = reverse(f(21)). Therefore 21 belongs to the sequence.
MATHEMATICA
rev[n_] := FromDigits[Reverse[IntegerDigits[n]]]; f[n_] := Prime[n]; Select[Range[10^5], f[rev[ # ]] == rev[f[ # ]] &]
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Joseph L. Pe, Apr 15 2002
EXTENSIONS
a(8) added by Ivan Neretin, May 30 2016
a(9) from Giovanni Resta, Apr 13 2017
STATUS
approved