A379938 - OEIS

%I #14 Jan 07 2025 09:52:04

%S 1,9,18,142,575,23652,3633466,10846595429,802467018101,

%T 2289255503212477

%N Numbers k such that the k-th prime is a power of two reversed.

%F A000040(a(n)) = A102385(n).

%F a(n) = A000720(A102385(n)). - _Michel Marcus_, Jan 07 2025

%e The 9th prime is 23, 23 reversed is 32, and 32 = 2^5, so 9 is a term.

%o (Python)

%o import sympy

%o for (k, p) in enumerate(sympy.primerange(10**8)):

%o     rev = int(str(p)[::-1])

%o     # is rev a power of two (or zero)?

%o     if rev & (rev - 1) == 0:

%o         print(k + 1, end=",")

%o print()

%Y Cf. A000040, A000079, A004087, A004094, A057708, A102385.

%K base,nonn,more

%O 1,2

%A _Kalle Siukola_, Jan 06 2025

%E a(8)-a(10) from _Amiram Eldar_, Jan 07 2025