A181615 -id:A181615 - cp's OEIS frontend

A175131 a(n) = 2^(n-1) mod semiprime(n).

1, 2, 4, 8, 2, 2, 1, 18, 6, 18, 1, 8, 1, 22, 4, 16, 23, 2, 14, 2, 52, 2, 49, 2, 10, 65, 64, 8, 44, 2, 64, 2, 42, 12, 62, 56, 31, 98, 81, 50, 74, 2, 1, 128, 96, 71, 60, 7, 81, 16, 94, 146, 1, 144, 108, 154, 133, 4, 46, 153, 67, 44, 88, 90, 198, 32, 14, 117

Offset: 1

Juri-Stepan Gerasimov, Dec 02 2010

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

Cf. A000079, A001358, A175036, A181615.

Module[{nn=300,sems,len},sems=Select[Range[nn],PrimeOmega[#]==2&];len = Length[sems];PowerMod[2,#[[1]]-1,#[[2]]]&/@Thread[{Range[len],sems}]] (* Harvey P. Dale, May 19 2018 *)

lista(nn) = {semip = select(n->bigomega(n) == 2, vector(nn, i, i)); for (i=1, #semip, print1(2^(i-1) % semip[i], ", "););} \\ Michel Marcus, Sep 11 2013

More terms from Michel Marcus, Sep 11 2013

A177266 Triangle read by rows: R(n,k) = 2^(semiprime(n)-semiprime(k)) mod semiprime(n).

Original entry on oeis.org

1, 4, 1, 5, 8, 1, 4, 6, 2, 1, 2, 4, 4, 2, 1, 8, 2, 4, 2, 2, 1, 11, 8, 1, 11, 2, 1, 1, 14, 20, 8, 4, 14, 18, 2, 1, 2, 13, 11, 18, 23, 24, 16, 8, 1, 10, 22, 6, 16, 14, 20, 6, 16, 2, 1, 17, 29, 16, 8, 17, 25, 4, 2, 25, 29, 1, 30, 16, 2, 18, 16, 8, 32, 16, 2, 18, 2, 1, 23, 32, 4, 2, 22

Offset: 1

Juri-Stepan Gerasimov, Dec 10 2010

			Triangle begins:
1,
4, 1,
5, 8, 1,
4, 6, 2, 1,
2, 4, 4, 2, 1,
8, 2, 4, 2, 2, 1,

Cf. A000079, A001358, A173622, A174479, A176066, A181615.

Table[PowerMod[2, #[[n]] - #[[;;n]], #[[n]]], {n, Length[#]}] & [Select[Range[50], PrimeOmega[#] == 2 &]] (* Paolo Xausa, Jun 28 2024 *)

trg(nn) = {semip = select(n->bigomega(n) == 2, vector(nn, i, i)); for (n = 1, #semip, for (k = 1, n, print1(2^(semip[n] - semip[k]) % semip[n], ", ");); print(););} \\ Michel Marcus, Sep 11 2013

Corrected by D. S. McNeil, Dec 10 2010

cp's OEIS Frontend

A175131 a(n) = 2^(n-1) mod semiprime(n).

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