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 A217559 Terms as well as ending/starting digits have alternating primality; this is the lexicographically earliest injective sequence with this property. 4
 1, 2, 4, 3, 6, 5, 8, 7, 9, 23, 10, 29, 22, 11, 24, 31, 25, 13, 12, 17, 14, 37, 15, 19, 26, 53, 16, 59, 28, 71, 30, 73, 18, 79, 20, 211, 21, 223, 48, 227, 49, 229, 27, 41, 32, 43, 44, 233, 45, 47, 46, 239, 33, 61, 34, 241, 35, 67, 60, 251, 36, 257, 62, 83, 63, 89, 38, 263, 64, 269, 39, 271, 50, 277 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Exactly every other term is prime; moreover the ending digit of a(n) and the initial digit of a(n+1) are never both prime or both composite. LINKS E. Angelini, Non-primes/primes: integers and digits alternate, SeqFan mailing list, Oct 06 2012 PROG (PARI) A217559(n, show_all=0, a=1, u)={for( i=2, n, u+=1<

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