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A124231
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Numbers k such that pi(k) is palindromic, where pi(k) is the number of primes less than or equal to k.
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1
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1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 31, 32, 33, 34, 35, 36, 79, 80, 81, 82, 137, 138, 193, 194, 195, 196, 257, 258, 259, 260, 261, 262
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OFFSET
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1,2
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LINKS
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EXAMPLE
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Every number from 1 to 28 inclusive belongs to this sequence as the number of primes less than or equal to k, for k <= 28, is a one-digit number, which is a palindrome.
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MATHEMATICA
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Select[Range[300], Reverse[IntegerDigits[PrimePi[ # ]]] == IntegerDigits[PrimePi[ # ]] &]
Position[PrimePi[Range[300]], _?(#==IntegerReverse[#]&)]//Flatten (* The program uses the IntegerReverse function from Mathematica version 10 *) (* Harvey P. Dale, Mar 02 2016 *)
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CROSSREFS
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KEYWORD
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base,nonn
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AUTHOR
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STATUS
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approved
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