A167735 Primes that become squares when prefixed with a 2.

5, 89, 401, 601, 809, 1609, 2801, 4649, 5281, 6569, 9241, 9929, 14369, 18089, 19961, 21841, 27529, 33289, 41081, 43049, 51001, 61121, 67289, 73529, 79841, 84089, 92681, 94849

Offset: 1

Author

Claudio Meller, Nov 10 2009

Keywords

• base
• nonn
• changed

Links

• Vincenzo Librandi and David A. Corneth, Table of n, a(n) for n = 1..10488 (First 222 terms from Vincenzo Librandi)

Crossrefs

Cf. A167734, A167736, A167737, A167738, A167739, A167740, A167741.

Programs

Mathematica

Select[Prime[Range[10000]],IntegerQ[Sqrt[FromDigits[Join[{2},IntegerDigits[#]]]]]&] (* Harvey P. Dale, Dec 16 2010 *)

PARI

\\ terms upto 10^n
upto(n) = {my(sqrt21 = sqrt(2.1), sqrt30 = sqrt(3.0),r,t=0);
for(k=2, n, for(j = ceil((sqrt21 * sqrt(10^k))), floor(sqrt30 * sqrt(10^k)),
r = j^2%10^k; if(isprime(r),t++;print1(r", "))));t} \\ David A. Corneth, Jun 24 2016