login
A247483
a(1)=4; for n>=1, a(n+1) is the smallest palindromic semiprime with a(n) as a central substring.
3
4, 141, 41414, 1414141, 314141413, 33141414133, 3331414141333, 14333141414133341, 121433314141413334121, 1012143331414141333412101, 17101214333141414133341210171, 3171012143331414141333412101713, 19317101214333141414133341210171391
OFFSET
1,1
EXAMPLE
a(1) = 4 = 2*2;
a(2) = 141 = 3*47;
a(3) = 41414 = 2*20707;
a(4) = 1414141 = 43*32887.
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 *)
CROSSREFS
Sequence in context: A299061 A299722 A210831 * A293752 A299833 A231949
KEYWORD
nonn,base,changed
AUTHOR
Michel Lagneau, Dec 01 2014
STATUS
approved