A089749 a(n) = smallest m > a(n-1) such that concatenation nm is a prime.

1, 3, 7, 19, 21, 31, 33, 39, 41, 49, 51, 59, 61, 71, 79, 93, 107, 119, 121, 123, 139, 147, 159, 169, 171, 177, 179, 181, 191, 197, 219, 233, 247, 253, 257, 263, 273, 281, 293, 343, 351, 359, 391, 417, 427, 439, 441, 449, 451, 459, 461, 489, 503, 517, 529, 531

Offset

1,2

Comments

Up to a(10^6), a(n) =~ 100000 + 24n + 10^(-6*n), which is almost a straight line. - Robert G. Wilson v

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Example

a(5) = 21 because the first prime after a(4) = 419 beginning with 5 is 521.

Maple

A[1]:= 1:
for n from 2 to 100 do
  for m from A[n-1]+1 do
    if isprime(ncat(n, m)) then A[n]:= m; break fi
od od:
seq(A[i], i=1..100); # Robert Israel, Feb 14 2021

Mathematica

a[1] = 1; a[1] = 1; a[n_] := a[n] = Block[{k = IntegerDigits[n], l = a[n - 1] + 2}, While[ !PrimeQ[ FromDigits[ Join[k, IntegerDigits[l]]]], l += 2]; l]; Table[ a[n], {n, 1, 55}]

Crossrefs

Sequence in context: A365139 A242170 A032675 * A032667 A109991 A248219

Adjacent sequences: A089746 A089747 A089748 * A089750 A089751 A089752

Keyword

base,nonn

Author

Amarnath Murthy, Nov 20 2003

Extensions

Corrected and extended by Robert G. Wilson v, Dec 05 2003

Status

approved