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 A068877 Largest n-digit prime with property that digits alternate in parity. 2
 7, 89, 983, 8969, 98981, 898987, 9898921, 89898983, 989898989, 8989898969, 98989898981, 898989898987, 9898989898901, 89898989898967, 989898989898943, 8989898989898969, 98989898989898981, 898989898989898943, 9898989898989898789 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Giovanni Resta, Table of n, a(n) for n = 1..500 EXAMPLE a(4) = 8969 as 8, 9, 6 and 9 have even and odd parity alternately. PROG (Sage) concat = lambda x: Integer(''.join(map(str, x)), base=10) def A068877(n): ....dd = {0:range(0, 10, 2)[::-1], 1: range(1, 10, 2)[::-1]} ....for d0 in [1..9][::-1]: ........if n % 2 == 0 and d0 % 2 == 1: continue # optimization ........ds = [dd[(d0+1+i) % 2] for i in range(n-1)] ........for dr in CartesianProduct(*ds): ............c = concat([d0]+dr) ............if is_prime(c): return c  # [D. S. McNeil, Apr 02 2011] CROSSREFS Cf. A030144, A068876. Sequence in context: A068692 A178006 A053302 * A121044 A107109 A089771 Adjacent sequences:  A068874 A068875 A068876 * A068878 A068879 A068880 KEYWORD nonn,base AUTHOR Amarnath Murthy, Mar 19 2002 EXTENSIONS a(15)-a(19) from Donovan Johnson, Apr 01 2011 STATUS approved

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Last modified January 19 05:21 EST 2019. Contains 319304 sequences. (Running on oeis4.)