A282183 Numbers k such that the reverse of the first k digits in the decimal expansion of Pi forms a prime.

1, 2, 5, 6, 7, 11, 15, 712, 7599, 13280, 13281, 21598, 23233

Offset: 1

Author

XU Pingya, Feb 13 2017

Keywords

• nonn
• more
• base
• less

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).

a(14) > 50000. - Michael S. Branicky, Feb 06 2025

Examples

			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
		

Links

• Shyam Sunder Gupta, Mystery of pi, Exploring the Beauty of Fascinating Numbers, Springer (2025) Ch. 19, 473-497.

Crossrefs

Cf. A092845, A007523, A011545.

Programs

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 *)

Extensions

a(11)-a(13) from Michael S. Branicky, Feb 06 2025