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 A289994 a(1) = 1, a(2) = 2, a(n) = smallest number not occurring earlier such that the concatenation a(n-2), a(n-1) and a(n) is a prime. 1
 1, 2, 7, 11, 19, 13, 9, 21, 43, 3, 31, 49, 17, 37, 29, 41, 27, 39, 67, 33, 81, 53, 51, 23, 89, 91, 59, 47, 73, 61, 87, 99, 79, 63, 117, 113, 77, 127, 103, 93, 111, 71, 69, 101, 147, 57, 97, 133, 119, 121, 139, 131, 107, 157, 151, 123, 153, 83, 143, 169, 109, 137 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Seiichi Manyama, Table of n, a(n) for n = 1..1000 EXAMPLE 127, 2711, 71119, 111913, ... are primes. MAPLE P:= proc(q) local a, b, j, k, n, v; a:=1; b:=2; v:={a, b}; j:=[op(v)]; for k from 1 to q do for n from 3 to q do if isprime(a*10^(ilog10(b)+ilog10(n)+2)+b*10^(ilog10(n)+1)+n) and v intersect {n}={} then v:=v union {n}; a:=b; b:=n; j:=[op(j), n]; break; fi; od; od; print(op(j)); end: P(170); # Paolo P. Lava, Jun 22 2018 CROSSREFS Cf. A075607, A073676, A083754, A110772. Sequence in context: A273600 A103776 A260889 * A040154 A166862 A105881 Adjacent sequences:  A289991 A289992 A289993 * A289995 A289996 A289997 KEYWORD nonn,base AUTHOR Seiichi Manyama, Sep 02 2017 STATUS approved

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Last modified July 17 18:47 EDT 2019. Contains 325109 sequences. (Running on oeis4.)