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

1,1

Links

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Example

249 is in the sequence because 49 = 7*7.

Mathematica

Select[Range[400], PrimeOmega[#]==PrimeOmega[Mod[#, 10^(IntegerLength[ #]-1)]] == 2&] (* Harvey P. Dale, Jun 07 2017 *)

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(500); for(n=1, #t, if(issemiprime(delleft(t[n])), print1(t[n], ", ")))

Crossrefs

Cf. A235687, A235689.

Cf. A227919, A227916, A069686.

Sequence in context: A323030 A005277 A079702 * A176274 A344872 A240227

Adjacent sequences: A235685 A235686 A235687 * A235689 A235690 A235691

Keyword

nonn,base

Author

Colin Barker, Jan 14 2014

Status

approved