A065725 Primes p such that the decimal expansion of its base-7 conversion is also prime.
2, 3, 5, 17, 29, 31, 43, 59, 71, 127, 157, 197, 211, 227, 239, 241, 337, 353, 367, 379, 409, 463, 491, 563, 577, 619, 647, 743, 757, 773, 787, 857, 911, 953, 967, 1093, 1123, 1163, 1193, 1249, 1303, 1373, 1429, 1459, 1471, 1499, 1583, 1597, 1613, 1627, 1669
Offset: 1
Examples
E.g., 787_10 = 2203_7 is prime, and so is 2203_10.
Links
- Harry J. Smith, Table of n, a(n) for n = 1..1000
Programs
-
Mathematica
Select[ Range[2500], PrimeQ[ # ] && PrimeQ[ FromDigits[ IntegerDigits[ #, 7]]] & ] Select[Prime[Range[300]],PrimeQ[FromDigits[IntegerDigits[#,7]]]&] (* Harvey P. Dale, Nov 10 2022 *)
-
PARI
isok(p) = isprime(p) && isprime(fromdigits(digits(p, 7), 10)); \\ Michel Marcus, Mar 05 2022
Comments