A124231 Numbers k such that pi(k) is palindromic, where pi(k) is the number of primes less than or equal to k.

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

Offset

1,2

Links

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Example

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.

Mathematica

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 *)

Crossrefs

Sequence in context: A197640 A246099 A280865 * A352800 A154470 A194907

Adjacent sequences: A124228 A124229 A124230 * A124232 A124233 A124234

Keyword

base,nonn

Author

Tanya Khovanova, Dec 13 2006

Status

approved