A175188 Composite numbers of the form k*10^k + 1.

201, 40001, 500001, 6000001, 70000001, 800000001, 100000000001, 1100000000001, 12000000000001, 130000000000001, 1400000000000001, 15000000000000001, 160000000000000001, 1700000000000000001, 18000000000000000001, 190000000000000000001, 2000000000000000000001

Offset: 1

Author

Michel Lagneau, Mar 01 2010

Keywords

• nonn

Examples

			For k=2, 2*10^2 + 1 = 201 = 3*67.
For k=4, 4*10^4 + 1 = 40001 = 13*17*181.
		

References

• M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 844.

Links

• M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy].

Crossrefs

Intersection of A002808 and A064748.

Subsequence of A005381.

Cf. A216347.

Programs

Maple

with(numtheory):for k from 1 to 100 do: n:=k*10^k +1:if type(n,prime) = false then print(n):else fi:od:

Mathematica

Select[Table[k*10^k+1, {k, 1, 20}], CompositeQ] (* Amiram Eldar, Aug 14 2023 *)

Extensions

a(16)-a(17) added and name corrected by Amiram Eldar, Aug 14 2023