A235688 Semiprimes which remain semiprimes when the leftmost digit is removed.
14, 26, 34, 39, 46, 49, 69, 74, 86, 94, 106, 115, 121, 122, 133, 134, 146, 155, 158, 169, 177, 185, 187, 194, 206, 209, 214, 215, 221, 226, 235, 249, 262, 265, 274, 287, 291, 295, 309, 314, 321, 326, 334, 335, 339, 346, 355, 358, 362, 365, 377, 382, 386, 391
Offset: 1
Examples
249 is in the sequence because 49 = 7*7.
Links
- Harvey P. Dale, Table of n, a(n) for n = 1..1000
Programs
-
Mathematica
Select[Range[400],PrimeOmega[#]==PrimeOmega[Mod[#,10^(IntegerLength[ #]-1)]] == 2&] (* Harvey P. Dale, Jun 07 2017 *)
-
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(500); for(n=1, #t, if(issemiprime(delleft(t[n])), print1(t[n],", ")))