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A282183
Numbers k such that the reverse of the first k digits in the decimal expansion of Pi forms a prime.
0
1, 2, 5, 6, 7, 11, 15, 712, 7599, 13280
OFFSET
1,2
COMMENTS
The initial digits of a few corresponding primes are in A007523. The last one a(10)=768556......62951413 is a prime with 13280-digit. That is A092845(13279).
EXAMPLE
1 is a term as the first digit of pi, 3, reversed is prime. 2 is a term as the first two digits of pi, 31, reversed is prime. 3 is not a term as the first three digits of pi, 314, reversed, is not prime. - David A. Corneth, Feb 13 2017
MATHEMATICA
Do[If[PrimeQ[FromDigits[Reverse[IntegerDigits[Floor[Pi*10^(n - 1)]]]]], Print[n]], {n, 13335}]
Module[{pid=RealDigits[Pi, 10, 20000][[1]]}, Select[Range[16000], PrimeQ[ FromDigits[ Reverse[Take[pid, #]]]]&]] (* Harvey P. Dale, Sep 06 2019 *)
CROSSREFS
KEYWORD
nonn,more,base,less
AUTHOR
XU Pingya, Feb 13 2017
STATUS
approved