A235689 Semiprimes which remain semiprimes when the leftmost and rightmost digits are removed.

141, 142, 143, 145, 146, 161, 166, 169, 194, 247, 249, 262, 265, 267, 291, 295, 298, 299, 341, 346, 361, 362, 365, 391, 393, 394, 395, 398, 445, 446, 447, 466, 469, 493, 497, 542, 543, 545, 562, 565, 566, 591, 597, 649, 662, 667, 669, 694, 695, 697, 698, 699

Offset

1,1

Links

Table of n, a(n) for n=1..52.

Example

169 = 13^2 is in the sequence because 6 = 2*3.

Mathematica

Select[Range[100, 700], PrimeOmega[#]==PrimeOmega[FromDigits[ Rest[ Most[ IntegerDigits[ #]]]]] ==2&] (* Harvey P. Dale, Nov 22 2018 *)

Prog

(PARI)
list(lim)=my(v=List(), t); forprime(p=2, sqrt(lim), t=p; forprime(q=p, lim\t, listput(v, t*q))); vecsort(Vec(v)) \\ From A001358
delleft(a) = my(b, c); b=#Str(a); c=a\(10^(b-1)); a-c*(10^(b-1))
issemiprime(n) = n>0 && bigomega(n)==2
t=list(700); for(n=1, #t, if(issemiprime(delleft(t[n]\10)), print1(t[n], ", ")))

Crossrefs

Cf. A235687, A235688.

Cf. A227919, A227916, A069686.

Sequence in context: A036192 A364805 A186962 * A045935 A144799 A208179

Adjacent sequences: A235686 A235687 A235688 * A235690 A235691 A235692

Keyword

nonn,base

Author

Colin Barker, Jan 14 2014

Status

approved