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%I #16 Mar 08 2020 00:04:22
%S 7,89,983,8969,98981,898987,9898921,89898983,989898989,8989898969,
%T 98989898981,898989898987,9898989898901,89898989898967,
%U 989898989898943,8989898989898969,98989898989898981,898989898989898943,9898989898989898789
%N Largest n-digit prime with property that digits alternate in parity.
%H Giovanni Resta, <a href="/A068877/b068877.txt">Table of n, a(n) for n = 1..500</a>
%e a(4) = 8969 as 8, 9, 6 and 9 have even and odd parity alternately.
%o (Sage)
%o concat = lambda x: Integer(''.join(map(str,x)),base=10)
%o def A068877(n):
%o dd = {0:range(0,10,2)[::-1], 1: range(1,10,2)[::-1]}
%o for d0 in [1..9][::-1]:
%o if n % 2 == 0 and d0 % 2 == 1: continue # optimization
%o ds = [dd[(d0+1+i) % 2] for i in range(n-1)]
%o for dr in cartesian_product(ds):
%o c = concat([d0]+dr)
%o if is_prime(c): return c # [_D. S. McNeil_, Apr 02 2011]
%Y Cf. A030144, A068876.
%K nonn,base
%O 1,1
%A _Amarnath Murthy_, Mar 19 2002
%E a(15)-a(19) from _Donovan Johnson_, Apr 01 2011
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