A235687 -id:A235687 - cp's OEIS frontend

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

Colin Barker, Jan 14 2014

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

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

Cf. A235687, A235689.

Cf. A227919, A227916, A069686.

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

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],", ")))

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

Original entry on oeis.org

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

Colin Barker, Jan 14 2014

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

Cf. A235687, A235688.

Cf. A227919, A227916, A069686.

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

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],", ")))

cp's OEIS Frontend

A235688 Semiprimes which remain semiprimes when the leftmost digit is removed.

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