cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

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A363681 Sphenic numbers sandwiched between two squarefree semiprimes.

Original entry on oeis.org

186, 266, 322, 470, 518, 534, 582, 590, 670, 754, 790, 814, 894, 994, 1146, 1158, 1166, 1338, 1390, 1562, 1686, 1798, 1842, 1958, 2118, 2158, 2230, 2318, 2454, 2482, 2514, 2570, 2630, 2758, 2786, 2922, 2930, 2994, 3154, 3206, 3262, 3278, 3378, 3454, 3522, 3562, 3714, 3786, 3830, 3838, 3962, 3982
Offset: 1

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Author

Tanya Khovanova and Massimo Kofler, Jun 14 2023

Keywords

Comments

Sphenic numbers are numbers that are products of three distinct primes.
This sequence is different from A362811: sphenic numbers sandwiched between semiprimes, as semiprimes are products of two primes that might not be distinct.

Examples

			186 = 2*3*31 is a sphenic number sandwiched between 185 = 5*37 and 187 = 11*17, both of which are squarefree semiprimes. Thus, 186 is in this sequence.
290 = 2*5*29 is a sphenic number sandwiched between semiprimes 289 = 17*17 and 291 = 3*97, one of which is not squarefree. Thus, 290 is not in this sequence but in A362811.

Crossrefs

Cf. A006881, A007304, A362811

Programs

  • Mathematica
    Select[Range[4000], Transpose[FactorInteger[#]][[2]] == {1, 1, 1} && Transpose[FactorInteger[# - 1]][[2]] == {1, 1} && Transpose[FactorInteger[# + 1]][[2]] == {1, 1} &]
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