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 A075607 a(1) = 1, a(n) = smallest number not occurring earlier such that the concatenation a(n-1) and a(n) is a prime. 5
 1, 3, 7, 9, 11, 17, 21, 13, 19, 31, 37, 27, 29, 39, 23, 33, 43, 49, 51, 47, 59, 53, 81, 61, 63, 67, 79, 93, 41, 57, 83, 69, 71, 77, 89, 99, 73, 121, 97, 87, 103, 91, 127, 123, 113, 111, 109, 133, 117, 101, 107, 119, 129, 169, 151, 141, 131, 143, 137, 147, 139 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Almost certainly a permutation of A045572. - David W. Wilson, Jan 15 2005 LINKS Paolo P. Lava, Table of n, a(n) for n = 1..10000 MAPLE with(numtheory): P:= proc(q) local a, b, n, v; v:={1}; a:=1; print(a); while true do for n from 3 to q do b:=a*10^(ilog10(n)+1)+n; if isprime(b) and v intersect {n}={} then v:=v union {n}; a:=n; print(n); break; fi; od; od; end: P(10^6); # Paolo P. Lava, May 09 2017 MATHEMATICA a = {{1}}; Do[k = 2; While[Nand[! MemberQ[a, #], PrimeQ@ FromDigits@ Join[a[[n - 1]], #]] &@ Set[d, IntegerDigits@ k], k++]; AppendTo[a, d], {n, 2, 61}]; FromDigits /@ a (* Michael De Vlieger, May 08 2017 *) PROG (PARI) A075607(n, show=0, a=1, u=[])={for(n=2, n, show&&print1(a", "); u=setunion(u, [a]); while(#u>1&&u[2]==u[1]+2, u=u[2..-1]); forstep(k=u[1]+2, 9e9, 2, setsearch(u, k)&&next; isprime(eval(Str(a, k))) && (a=k) && break)); a} \\ Use 2nd, 3rd or 4th optional arg to print intermediate terms, to use another starting value or to exclude some terms. - M. F. Hasler, Nov 25 2015 CROSSREFS Cf. A075608, A075609, A075610. Sequence in context: A291348 A186890 A248667 * A256465 A275386 A275602 Adjacent sequences:  A075604 A075605 A075606 * A075608 A075609 A075610 KEYWORD base,nonn AUTHOR Amarnath Murthy, Sep 28 2002 EXTENSIONS More terms from Michel ten Voorde Jun 23 2003 STATUS approved

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Last modified September 17 07:51 EDT 2021. Contains 347478 sequences. (Running on oeis4.)