A247483 a(1)=4; for n>=1, a(n+1) is the smallest palindromic semiprime with a(n) as a central substring.

4, 141, 41414, 1414141, 314141413, 33141414133, 3331414141333, 14333141414133341, 121433314141413334121, 1012143331414141333412101, 17101214333141414133341210171, 3171012143331414141333412101713, 19317101214333141414133341210171391

Offset: 1

Author

Michel Lagneau, Dec 01 2014

Keywords

• nonn
• base

Examples

			a(1) = 4 = 2*2;
a(2) = 141 = 3*47;
a(3) = 41414 = 2*20707;
a(4) = 1414141 = 43*32887.
		

Crossrefs

Cf. A001358, A053600.

Programs

Mathematica

d[n_] := IntegerDigits[n]; t = {x = 4}; Do[i = 1; While[!PrimeOmega[y = FromDigits[Flatten[{z = d[i], d[x], Reverse[z]}]]]==2, i++]; AppendTo[t, x = y], {n, 12}]; t
nxt[n_]:=Module[{k=1},While[PrimeOmega[FromDigits[Join[IntegerDigits[k],IntegerDigits[n],Reverse[IntegerDigits[k]]]]]!=2,k++]; FromDigits[ Join[IntegerDigits[k],IntegerDigits[n],Reverse[IntegerDigits[k]]]]]; NestList[nxt,4,15] (* Harvey P. Dale, Oct 30 2024 *)