A062210 -id:A062210 - cp's OEIS frontend

A062209 Numbers k such that the smoothly undulating palindromic number (4*10^k-7)/33 = 121...21 is a prime (or PRP).

7, 11, 43, 139, 627, 1399, 1597, 1979, 7809, 14059, 46499

Offset: 1

Patrick De Geest and Hans Rosenthal (Hans.Rosenthal(AT)t-online.de), Jun 15 2001

Prime versus probable prime status and proofs are given in the author's table.
No further terms < 100000. - Ray Chandler, Aug 17 2011
The corresponding primes, called smoothly undulating palindromic primes (cf. links, A032758 and A059758), are listed in A092696. The number of '12's is given in A056803(n) = (a(n)-1)/2. - M. F. Hasler, Jul 30 2015

			k=11 --> (12*10^11 - 21)/99 = 12121212121.

J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 139, p. 48, Ellipses, Paris 2008.

Cf. A062210-A062232, A059758, A032758, A092696, A056803.

d[n_]:=IntegerDigits[n]; Length/@d[Select[NestList[FromDigits[Join[d[#],{2,1}]]&,1,1000],PrimeQ]] (* Jayanta Basu, May 25 2013 *)

for(n=1,1e5,ispseudoprime(5^n<<(n+2)\33)&&print1(n",")) \\ M. F. Hasler, Jul 30 2015

a(11) = 46499 from Ray Chandler, Nov 11 2010
Edited by Ray Chandler, Aug 17 2011
Name and other items edited by M. F. Hasler, Jul 30 2015

cp's OEIS Frontend

A062209 Numbers k such that the smoothly undulating palindromic number (4*10^k-7)/33 = 121...21 is a prime (or PRP).

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