A217559 Terms as well as ending/starting digits have alternating primality; this is the lexicographically earliest injective sequence with this property.

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

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

Table of n, a(n) for n=1..74.

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<<a; show_all & print1(a", "); for(t=1, 9e9, bittest(u, t) & next; isprime(t)==bittest(i, 0) & next; isprime(a%10) == isprime(t\10^(#Str(t)-1)) & (t+=10^(#Str(t)-1)-1) & next; a=t; break )); a}

Crossrefs

This is a simplified variant of A217560.

See also A217555, A217556, where "primality" is replaced by "parity".

Sequence in context: A067747 A073846 A110458 * A217560 A114112 A113981

Adjacent sequences: A217556 A217557 A217558 * A217560 A217561 A217562

Keyword

nonn,base

Author

Eric Angelini and M. F. Hasler, Oct 06 2012

Status

approved