A075607 a(1) = 1, a(n) = smallest number not occurring earlier such that the concatenation a(n-1) and a(n) is a prime.
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
Offset: 1
Links
- Paolo P. Lava, Table of n, a(n) for n = 1..10000
Programs
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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 *) -
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
Extensions
More terms from Michel ten Voorde Jun 23 2003
Comments