A338908 - OEIS

A338908

Squarefree semiprimes whose prime indices sum to an even number.

10, 21, 22, 34, 39, 46, 55, 57, 62, 82, 85, 87, 91, 94, 111, 115, 118, 129, 133, 134, 146, 155, 159, 166, 183, 187, 194, 203, 205, 206, 213, 218, 235, 237, 247, 253, 254, 259, 267, 274, 295, 298, 301, 303, 314, 321, 334, 335, 339, 341, 358, 365, 371, 377, 382

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OFFSET

1,1

COMMENTS

A squarefree semiprime is a product of any two distinct prime numbers. A prime index of n is a number m such that the m-th prime number divides n. The multiset of prime indices of n is row n of A112798.

LINKS

Table of n, a(n) for n=1..55.

EXAMPLE

The sequence of terms together with their prime indices begins:

10: {1,3} 115: {3,9} 213: {2,20}

21: {2,4} 118: {1,17} 218: {1,29}

22: {1,5} 129: {2,14} 235: {3,15}

34: {1,7} 133: {4,8} 237: {2,22}

39: {2,6} 134: {1,19} 247: {6,8}

46: {1,9} 146: {1,21} 253: {5,9}

55: {3,5} 155: {3,11} 254: {1,31}

57: {2,8} 159: {2,16} 259: {4,12}

62: {1,11} 166: {1,23} 267: {2,24}

82: {1,13} 183: {2,18} 274: {1,33}

85: {3,7} 187: {5,7} 295: {3,17}

87: {2,10} 194: {1,25} 298: {1,35}

91: {4,6} 203: {4,10} 301: {4,14}

94: {1,15} 205: {3,13} 303: {2,26}

111: {2,12} 206: {1,27} 314: {1,37}

MATHEMATICA

Select[Range[100], SquareFreeQ[#]&&PrimeOmega[#]==2&& EvenQ[Total[PrimePi/@First/@FactorInteger[#]]]&]

CROSSREFS

A031215 looks at primes instead of semiprimes.

A300061 and A319241 (squarefree) look all numbers (not just semiprimes).

A338905 has this as union of even-indexed rows.

A338906 is the nonsquarefree version.

A338907 is the odd version.

A001358 lists semiprimes, with odd/even terms A046315/A100484.

A005117 lists squarefree numbers.

A006881 lists squarefree semiprimes, with odd/even terms A046388/A100484.

A024697 is the sum of semiprimes of weight n.

A025129 is the sum of squarefree semiprimes of weight n.

A056239 gives the sum of prime indices of n.

A289182/A115392 list the positions of odd/even terms in A001358.

A320656 counts factorizations into squarefree semiprimes.

A332765 gives the greatest squarefree semiprime of weight n.

A338898, A338912, and A338913 give the prime indices of semiprimes, with product A087794, sum A176504, and difference A176506.

A338899, A270650, and A270652 give the prime indices of squarefree semiprimes, with difference A338900.

A338904 groups semiprimes by weight.

A338911 lists products of pairs of primes both of even index.

A339114/A339115 give the least/greatest semiprime of weight n.

A339116 groups squarefree semiprimes by greater prime factor.

Cf. A000040, A001221, A001222, A087112, A098350, A112798, A168472, A338901, A338904, A339004, A339005.

Sequence in context: A164712 A336558 A374897 * A226195 A181450 A364079

Adjacent sequences: A338905 A338906 A338907 * A338909 A338910 A338911

KEYWORD

nonn

AUTHOR

Gus Wiseman, Nov 28 2020

STATUS

approved