A069628 - OEIS

%I #5 Jan 01 2024 13:59:03

%S 1,11,211,2111,22111,2211109,92211109,9221110901,29221110901,

%T 2922111090137,32922111090137,3292211109013747,73292211109013747,

%U 7329221110901374771,157329221110901374771,15732922111090137477101

%N a(1) = 1; a(2n) = smallest prime starting (most significant digits) with a(2n-1). a(2n+1) = smallest prime ending (least significant digits) in a(2n).

%t a[n_] := (j = ToString[n]; l = {"1", "3", "7", "9", "01", "03", "07", "09"}; k = 1; While[p = ToExpression[ StringJoin[j, ToString[l[[k]] ]]]; k < 9 && ! PrimeQ[p], k++ ]; If[k < 9, Return[p]]; i = IntegerDigits[n]; k = 11; While[p = FromDigits[Join[i, IntegerDigits[k]]]; ! PrimeQ[p], k++ ]; Return[p]); b[n_] := (i = IntegerDigits[n]; k = 1; While[p = FromDigits[ Join[ IntegerDigits[k], i]]; !PrimeQ[p], k++ ]; Return[p]); f[1] = 1; f[n_] := If[ EvenQ[n], a[f[n - 1]], b[f[n - 1]]]; Table[ f[n], {n, 1, 18}]

%Y Cf. A069614 to A069621, A069628 to A069636.

%K nonn,base

%O 1,2

%A _Amarnath Murthy_, Mar 27 2002

%E Edited and extended by _Robert G. Wilson v_, Apr 03 2002