OFFSET
1,2
COMMENTS
a(2) = 13 and a(4) = 7069 are primes. What other terms are primes? - N. J. A. Sloane, Feb 18 2023
a(38) is the next prime. - Michael S. Branicky, Feb 18 2023
LINKS
FORMULA
a(n) = binary_to_decimal(concatenate(1,10,11,..., binary(n-2), binary(n-1), binary(n), binary(n-1), binary(n-2),..., 11, 10, 1))
EXAMPLE
a(1)=binary_to_decimal(1)=1, a(2)=binary_to_decimal(1101)=13, a(3)=binary_to_decimal(11011101)=221, a(4)=binary_to_decimal(1101110011101)=7069 etc.
MAPLE
a:= n-> Bits[Join](map(x-> Bits[Split](x)[], [$1..n, n-i$i=1..n-1])):
seq(a(n), n=1..16); # Alois P. Heinz, Feb 18 2023
PROG
(PARI) a(n)=sum(i=1, #n=concat(vector(n*2-1, k, binary(min(k, n*2-k)))), n[i]<<(#n-i))
(PARI) A173427(n)={my(s=0, s1=0, t=0, b=0); for(k=1, n-1, s1+=k<<t+=b; k>>b&&b++; s=s<<b+k); t+=b; n>>b&&b++; (s<<b+n)<<t+s1} \\ M. F. Hasler, Aug 06 2015
(Python)
from itertools import count, islice
def agen(): # generator of terms
sl, sr, sk = "", "", "1"
for k in count(1):
sk = bin(k)[2:]
sl += sk
yield int(sl + sr, 2)
sr = sk + sr
print(list(islice(agen(), 16))) # Michael S. Branicky, Feb 18 2023
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Umut Uludag, Feb 18 2010.
STATUS
approved