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A348792 Numbers k such that the reverse concatenation of the first k binary numbers A098780(k) is prime. 1
2, 3, 4, 7, 11, 13, 25, 97, 110, 1939 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
LINKS
EXAMPLE
a(4) = 7 is because the binary number 111 110 101 100 11 10 1 (with no spaces), which is 128413 in decimal, is prime.
MAPLE
q:= n-> isprime(Bits[Join](['Bits[Split](i)[]'$i=1..n])):
select(q, [$1..200])[]; # Alois P. Heinz, Dec 03 2021
MATHEMATICA
f[n_] := FromDigits[Flatten @ IntegerDigits[Range[n, 1, -1], 2], 2]; Select[Range[120], PrimeQ[f[#]] &] (* Amiram Eldar, Dec 03 2021 *)
PROG
(Python)
from sympy import isprime
def afind(limit):
s, k = "", 1
for k in range(1, limit+1):
s += bin(k)[2:][::-1]
t = int(s[::-1], 2)
if isprime(t):
print(k, end=", ")
afind(200) # Michael S. Branicky, Dec 03 2021
CROSSREFS
Sequence in context: A232543 A279071 A171027 * A064933 A060731 A238492
KEYWORD
nonn,base,hard,more
AUTHOR
N. J. A. Sloane, Dec 03 2021
EXTENSIONS
a(8)-a(10) from Amiram Eldar, Dec 03 2021
STATUS
approved

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Last modified March 29 08:13 EDT 2024. Contains 371265 sequences. (Running on oeis4.)