A345962 Numbers whose prime indices have alternating sum -2.

10, 21, 40, 55, 84, 90, 91, 160, 187, 189, 210, 220, 247, 250, 336, 360, 364, 391, 462, 490, 495, 525, 551, 640, 713, 748, 756, 810, 819, 840, 858, 880, 988, 1000, 1029, 1073, 1155, 1210, 1271, 1326, 1344, 1375, 1440, 1456, 1564, 1591, 1683, 1690, 1701, 1848

Offset

1,1

Comments

A prime index of n is a number m such that prime(m) divides n. The multiset of prime indices of n is row n of A112798.

The alternating sum of a sequence (y_1,...,y_k) is Sum_i (-1)^(i-1) y_i. Of course, the alternating sum of prime indices is also the reverse-alternating sum of reversed prime indices.

Also numbers with even Omega (A001222) and exactly two odd conjugate prime indices. The case of odd Omega is A345960, and the union is A345961.

Links

Table of n, a(n) for n=1..50.

Example

The initial terms and their prime indices:
   10: {1,3}
   21: {2,4}
   40: {1,1,1,3}
   55: {3,5}
   84: {1,1,2,4}
   90: {1,2,2,3}
   91: {4,6}
  160: {1,1,1,1,1,3}
  187: {5,7}
  189: {2,2,2,4}
  210: {1,2,3,4}
  220: {1,1,3,5}
  247: {6,8}
  250: {1,3,3,3}
  336: {1,1,1,1,2,4}
  360: {1,1,1,2,2,3}

Mathematica

primeMS[n_]:=If[n==1, {}, Flatten[Cases[FactorInteger[n], {p_, k_}:>Table[PrimePi[p], {k}]]]];
ats[y_]:=Sum[(-1)^(i-1)*y[[i]], {i, Length[y]}];
Select[Range[0, 100], ats[primeMS[#]]==-2&]

Crossrefs

Below we use k to indicate alternating sum.

The k = 0 version is A000290, counted by A000041.

The k = 1 version is A001105 (reverse: A345958).

The k > 0 version is A026424.

These are the positions of -2's in A316524.

These multisets are counted by A344741 (positive 2: A120452).

The k = -1 version is A345959.

The k = 2 version is A345960, counted by A000097.

A002054/A345924/A345923 count/rank compositions with alternating sum -2.

A056239 adds up prime indices, row sums of A112798.

A088218/A345925/A345922 count/rank compositions with alternating sum 2.

A097805 counts compositions by alternating (or reverse-alternating) sum.

A103919 counts partitions by sum and alternating sum (reverse: A344612).

A325534 and A325535 count separable and inseparable partitions.

A344606 counts alternating permutations of prime indices.

Cf. A000037, A000070, A001791, A027187, A028260, A239830, A341446, A344609, A344616, A344651, A345197, A345910, A345912, A345961.

Sequence in context: A324600 A240536 A060852 * A195142 A256884 A089584

Adjacent sequences: A345959 A345960 A345961 * A345963 A345964 A345965

Keyword

nonn

Author

Gus Wiseman, Jul 12 2021

Status

approved