cp's OEIS Frontend

A371666 Composite numbers which equal the reverse of the concatenation of their ascending ordered prime factors, with repetition, when written in binary.

%I A371666 #11 Apr 13 2024 14:57:40
%S A371666 623,78205,101239,80073085,1473273719
%N A371666 Composite numbers which equal the reverse of the concatenation of their ascending ordered prime factors, with repetition, when written in binary.
%C A371666 A subsequence of A355973. The first five numbers each have only two prime factors - do terms exist with three or more?
%e A371666 101239 is a term as 101239_10 = 29_10 * 3491_10 = 11101_2 * 110110100011_2 = "11101110110100011"_2 which when reversed is "11000101101110111"_2 = 101239_10.
%o A371666 (Python)
%o A371666 from itertools import count, islice
%o A371666 from sympy import factorint
%o A371666 def A371666_gen(startvalue=4): # generator of terms >= startvalue
%o A371666     for n in count(max(startvalue,4)):
%o A371666         f = factorint(n)
%o A371666         if sum(f.values()) > 1:
%o A371666             c = 0
%o A371666             for p in sorted(f,reverse=True):
%o A371666                 a, q = p.bit_length(), int(bin(p)[:1:-1],2)
%o A371666                 for _ in range(f[p]):
%o A371666                     c = (c<<a)+q
%o A371666             if c == n:
%o A371666                 yield n
%o A371666 A371666_list = list(islice(A371666_gen(),3)) # _Chai Wah Wu_, Apr 13 2024
%Y A371666 Cf. A355973, A371821, A027750, A009944.
%K A371666 nonn,base,more
%O A371666 1,1
%A A371666 _Scott R. Shannon_, Apr 02 2024