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%I #22 Jul 25 2023 07:17:55
%S 1,2,3,2,5,3,7,2,2,5,11,3,13,7,7,2,17,2,19,5,5,11,23,3,3,13,13,7,29,7,
%T 31,2,2,17,17,2,37,19,19,5,41,5,43,11,11,23,47,3,3,3,3,13,53,13,13,7,
%U 7,29,59,7,61,31,31,2,2,2,67,17,17,17,71,2,73,37,37,19,19,19,79,5,5
%N Fixed point of "n -> n/2 or (n-1)/2 until result is prime".
%C a(n) is the largest prime whose binary expansion is an initial substring of n's binary expansion. - _Charlie Neder_, Oct 27 2018
%C a(1) = 1 by convention. - _David A. Corneth_, Oct 27 2018
%H Reinhard Zumkeller, <a href="/A039634/b039634.txt">Table of n, a(n) for n = 1..10000</a>
%t ner[ n_Integer ] := FixedPoint[ If[ EvenQ[ # ]&&#>2, #/2, If[ PrimeQ[ # ]||(#=== 1), #, (#-1)/2 ] ]&, n, 20 ]
%o (Haskell)
%o a039634 1 = 1
%o a039634 n = until ((== 1) . a010051) (flip div 2) n
%o -- _Reinhard Zumkeller_, Nov 17 2013
%o (PARI) a(n)=while(n>3 && !isprime(n), n\=2); n \\ _Charles R Greathouse IV_, Jun 23 2017
%o (Python)
%o from sympy import isprime
%o def a(n):
%o while n>1 and not isprime(n): n>>=1
%o return n
%o print([a(n) for n in range(1, 82)]) # _Michael S. Branicky_, Jul 24 2023
%Y Cf. A039635-A039645, A010051, A039636, A039638, A039639.
%K nonn,easy,nice
%O 1,2
%A _Wouter Meeussen_
%E Offset corrected by _Reinhard Zumkeller_, Nov 17 2013
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