A243055 -id:A243055 - cp's OEIS frontend

A372589 Numbers k > 1 such that (greatest binary index of k) + (greatest prime index of k) is even.

3, 4, 5, 9, 12, 13, 14, 16, 17, 20, 22, 23, 25, 30, 31, 35, 36, 37, 38, 39, 42, 43, 48, 49, 52, 53, 54, 56, 57, 58, 61, 63, 64, 66, 67, 68, 69, 73, 75, 77, 80, 82, 83, 85, 88, 90, 92, 93, 94, 97, 99, 100, 102, 103, 109, 110, 115, 118, 119, 120, 121, 123, 124

Offset: 1

Gus Wiseman, May 14 2024

nonn

A binary index of n is any position of a 1 in its reversed binary expansion. The binary indices of n are row n of A048793.
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 odd version is A372588.

			The terms (center), their binary indices (left), and their weakly decreasing prime indices (right) begin:
        {1,2}   3  (2)
          {3}   4  (1,1)
        {1,3}   5  (3)
        {1,4}   9  (2,2)
        {3,4}  12  (2,1,1)
      {1,3,4}  13  (6)
      {2,3,4}  14  (4,1)
          {5}  16  (1,1,1,1)
        {1,5}  17  (7)
        {3,5}  20  (3,1,1)
      {2,3,5}  22  (5,1)
    {1,2,3,5}  23  (9)
      {1,4,5}  25  (3,3)
    {2,3,4,5}  30  (3,2,1)
  {1,2,3,4,5}  31  (11)
      {1,2,6}  35  (4,3)
        {3,6}  36  (2,2,1,1)
      {1,3,6}  37  (12)
      {2,3,6}  38  (8,1)
    {1,2,3,6}  39  (6,2)
      {2,4,6}  42  (4,2,1)
    {1,2,4,6}  43  (14)

For sum (A372428, zeros A372427) we have A372587, complement A372586.
For minimum (A372437) we have A372440, complement A372439.
For length (A372441, zeros A071814) we have A372591, complement A372590.
Positions of even terms in A372442, zeros A372436.
The complement is A372588.
For just binary indices:
- length: A001969, complement A000069
- sum: A158704, complement A158705
- minimum:  A036554, complement A003159
- maximum: A053754, complement A053738
For just prime indices:
- length: A026424 A028260 (count A027187), complement (count A027193)
- sum: A300061 (count A058696), complement A300063 (count A058695)
- minimum: A340933 (count A026805), complement A340932 (count A026804)
- maximum: A244990 (count A027187), complement A244991 (count A027193)
A019565 gives Heinz number of binary indices, adjoint A048675.
A029837 gives greatest binary index, least A001511.
A031215 lists even-indexed primes, odd A031368.
A048793 lists binary indices, length A000120, reverse A272020, sum A029931.
A061395 gives greatest prime index, least A055396.
A070939 gives length of binary expansion.
A112798 lists prime indices, length A001222, reverse A296150, sum A056239.
Cf. A000720, A006141, A066207, A243055, A257991, A300272, A304818, A340604, A341446, A372429-A372433, A372438.

Select[Range[2,100],EvenQ[IntegerLength[#,2]+PrimePi[FactorInteger[#][[-1,1]]]]&]

Numbers k such that A070939(k) + A061395(k) is even.

A325133 Heinz number of the integer partition obtained by removing the inner lining, or, equivalently, the largest hook, of the integer partition with Heinz number n.

Original entry on oeis.org

1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 1, 3, 1, 4, 1, 1, 2, 1, 1, 2, 1, 3, 2, 1, 1, 2, 1, 1, 2, 1, 1, 4, 1, 1, 1, 5, 3, 2, 1, 1, 4, 3, 1, 2, 1, 1, 2, 1, 1, 4, 1, 3, 2, 1, 1, 2, 3, 1, 2, 1, 1, 6, 1, 5, 2, 1, 1, 8, 1, 1, 2, 3, 1, 2, 1, 1, 4, 5, 1, 2, 1, 3, 1, 1, 5, 4, 3, 1, 2, 1, 1, 6

Offset: 1

Gus Wiseman, Apr 02 2019

nonn

The Heinz number of an integer partition (y_1,...,y_k) is prime(y_1)*...*prime(y_k).

			The partition with Heinz number 715 is (6,5,3), with diagram
  o o o o o o
  o o o o o
  o o o
which has inner lining
          o o
      o o o
  o o o
or largest hook
  o o o o o o
  o
  o
both of which have complement
  o o o o
  o o
which is the partition (4,2) with Heinz number 21, so a(715) = 21.

Antti Karttunen, Table of n, a(n) for n = 1..20000
Antti Karttunen, Data supplement: n, a(n) computed for n = 1..65537

Positions of ones are A093641 (Heinz numbers of hooks). The number of iterations required to reach 1 starting with n is A257990(n).

Cf. A000720, A001222, A046660, A052126, A056239, A061395, A064989, A112798, A243055, A252464, A325134, A325135.

primeMS[n_]:=If[n==1,{},Flatten[Cases[FactorInteger[n],{p_,k_}:>Table[PrimePi[p],{k}]]]];
Table[If[n==1,1,Times@@Prime/@DeleteCases[Most[primeMS[n]]-1,0]],{n,100}]

A052126(n) = if(1==n,n,n/vecmax(factor(n)[, 1]));
A064989(n) = { my(f); f = factor(n); if((n>1 && f[1,1]==2), f[1,2] = 0); for (i=1, #f~, f[i,1] = precprime(f[i,1]-1)); factorback(f) };
A325133(n) = A052126(A064989(n)); \\ Antti Karttunen, Apr 14 2019

a(n) = A064989(A052126(n)) = A052126(A064989(n)).

More terms from Antti Karttunen, Apr 14 2019

A372439 Numbers k such that the least binary index of k plus the least prime index of k is odd.

Original entry on oeis.org

2, 3, 6, 7, 8, 9, 10, 13, 14, 15, 18, 19, 21, 22, 24, 26, 27, 29, 30, 32, 33, 34, 37, 38, 39, 40, 42, 43, 45, 46, 49, 50, 51, 53, 54, 56, 57, 58, 61, 62, 63, 66, 69, 70, 71, 72, 74, 75, 77, 78, 79, 81, 82, 86, 87, 88, 89, 90, 91, 93, 94, 96, 98, 99, 101, 102

Offset: 1

Gus Wiseman, May 06 2024

A binary index of n is any position of a 1 in its reversed binary expansion. The binary indices of n are row n of A048793.

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 terms (center), their binary indices (left), and their prime indices (right) begin:
        {2}   2  (1)
      {1,2}   3  (2)
      {2,3}   6  (2,1)
    {1,2,3}   7  (4)
        {4}   8  (1,1,1)
      {1,4}   9  (2,2)
      {2,4}  10  (3,1)
    {1,3,4}  13  (6)
    {2,3,4}  14  (4,1)
  {1,2,3,4}  15  (3,2)
      {2,5}  18  (2,2,1)
    {1,2,5}  19  (8)
    {1,3,5}  21  (4,2)
    {2,3,5}  22  (5,1)
      {4,5}  24  (2,1,1,1)
    {2,4,5}  26  (6,1)
  {1,2,4,5}  27  (2,2,2)
  {1,3,4,5}  29  (10)
  {2,3,4,5}  30  (3,2,1)
        {6}  32  (1,1,1,1,1)
      {1,6}  33  (5,2)
      {2,6}  34  (7,1)

Positions of odd terms in A372437.
The complement is 1 followed by A372440.
For sum (A372428, zeros A372427) we have A372586, complement A372587.
For maximum (A372442, zeros A372436) we have A372588, complement A372589.
For length (A372441, zeros A071814) we have A372590, complement A372591.
A003963 gives product of prime indices, binary A096111.
A019565 gives Heinz number of binary indices, adjoint A048675.
A029837 gives greatest binary index, least A001511.
A048793 lists binary indices, length A000120, reverse A272020, sum A029931.
A061395 gives greatest prime index, least A055396.
A070939 gives length of binary expansion.
A112798 lists prime indices, length A001222, reverse A296150, sum A056239.
Cf. A000720, A061712, A174090, A243055, A359495, A372429, A372430, A372431, A372432, A372438, A372471.

bix[n_]:=Join@@Position[Reverse[IntegerDigits[n,2]],1];
prix[n_]:=If[n==1,{},Flatten[Cases[FactorInteger[n],{p_,k_}:>Table[PrimePi[p],{k}]]]];
Select[Range[100],OddQ[Min[bix[#]]+Min[prix[#]]]&]

A372440 Numbers k such that the least binary index of k plus the least prime index of k is even.

Original entry on oeis.org

4, 5, 11, 12, 16, 17, 20, 23, 25, 28, 31, 35, 36, 41, 44, 47, 48, 52, 55, 59, 60, 64, 65, 67, 68, 73, 76, 80, 83, 84, 85, 92, 95, 97, 100, 103, 108, 109, 112, 115, 116, 121, 124, 125, 127, 132, 137, 140, 143, 144, 145, 148, 149, 155, 156, 157, 164, 167, 172

Offset: 1

Gus Wiseman, May 06 2024

A binary index of n is any position of a 1 in its reversed binary expansion. The binary indices of n are row n of A048793.

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 terms (center), their binary indices (left), and their prime indices (right) begin:
          {3}   4  (1,1)
        {1,3}   5  (3)
      {1,2,4}  11  (5)
        {3,4}  12  (2,1,1)
          {5}  16  (1,1,1,1)
        {1,5}  17  (7)
        {3,5}  20  (3,1,1)
    {1,2,3,5}  23  (9)
      {1,4,5}  25  (3,3)
      {3,4,5}  28  (4,1,1)
  {1,2,3,4,5}  31  (11)
      {1,2,6}  35  (4,3)
        {3,6}  36  (2,2,1,1)
      {1,4,6}  41  (13)
      {3,4,6}  44  (5,1,1)
  {1,2,3,4,6}  47  (15)
        {5,6}  48  (2,1,1,1,1)
      {3,5,6}  52  (6,1,1)
  {1,2,3,5,6}  55  (5,3)
  {1,2,4,5,6}  59  (17)
    {3,4,5,6}  60  (3,2,1,1)
          {7}  64  (1,1,1,1,1,1)

For sum (A372428, zeros A372427) we have A372587, complement A372586.
Positions of even terms in A372437.
The complement is 1 followed by A372439.
For length (A372441, zeros A071814) we have A372591, complement A372590.
For maximum (A372442, zeros A372436) we have A372589, complement A372588.
A003963 gives product of prime indices, binary A096111.
A019565 gives Heinz number of binary indices, adjoint A048675.
A029837 gives greatest binary index, least A001511.
A048793 lists binary indices, length A000120, reverse A272020, sum A029931.
A061395 gives greatest prime index, least A055396.
A070939 gives length of binary expansion.
A112798 lists prime indices, length A001222, reverse A296150, sum A056239.
Cf. A000720, A061712, A174090, A243055, A359402, A359495, A372429, A372430, A372431, A372432, A372438, A372471.

bix[n_]:=Join@@Position[Reverse[IntegerDigits[n,2]],1];
prix[n_]:=If[n==1,{},Flatten[Cases[FactorInteger[n],{p_,k_}:>Table[PrimePi[p],{k}]]]];
Select[Range[100],EvenQ[Min[bix[#]]+Min[prix[#]]]&]

A355532 Maximal augmented difference between adjacent reversed prime indices of n; a(1) = 0.

Original entry on oeis.org

0, 1, 2, 1, 3, 2, 4, 1, 2, 3, 5, 2, 6, 4, 2, 1, 7, 2, 8, 3, 3, 5, 9, 2, 3, 6, 2, 4, 10, 2, 11, 1, 4, 7, 3, 2, 12, 8, 5, 3, 13, 3, 14, 5, 2, 9, 15, 2, 4, 3, 6, 6, 16, 2, 3, 4, 7, 10, 17, 2, 18, 11, 3, 1, 4, 4, 19, 7, 8, 3, 20, 2, 21, 12, 2, 8, 4, 5, 22, 3, 2

Offset: 1

Gus Wiseman, Jul 14 2022

nonn

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 augmented differences aug(q) of a (usually weakly decreasing) sequence q of length k are given by aug(q)i = q_i - q{i+1} + 1 if i < k and aug(q)_k = q_k. For example, we have aug(6,5,5,3,3,3) = (2,1,3,1,1,3).

			The reversed prime indices of 825 are (5,3,3,2), with augmented differences (3,1,2,2), so a(825) = 3.

Gus Wiseman, Sequences counting and ranking integer partitions by the differences of their successive parts.

Crossrefs found in the link are not repeated here.
Prepending 1 to the positions of 1's gives A000079.
Positions of first appearances are A008578.
Positions of 2's are A065119.
The non-augmented version is A286470, also A355526.
The non-augmented minimal version is A355524, also A355525.
The minimal version is A355531.
Row maxima of A355534, which has Heinz number A325351.
A001222 counts prime indices, distinct A001221.
A112798 lists prime indices, sum A056239.
Cf. A066312, A124010, A129654, A243055, A243056, A307824, A325366, A325394, A355533, A355536.

primeMS[n_]:=If[n==1,{},Flatten[Cases[FactorInteger[n],{p_,k_}:>Table[PrimePi[p],{k}]]]];
aug[y_]:=Table[If[i

A356958 Triangle read by rows: if n has weakly increasing prime indices (a,b,...,y,z) then row n is (b-a+1, ..., y-a+1, z-a+1).

Original entry on oeis.org

1, 2, 1, 1, 1, 3, 1, 2, 4, 2, 1, 1, 1, 2, 2, 1, 3, 3, 5, 1, 1, 2, 1, 6, 1, 1, 1, 4, 2, 3, 1, 1, 1, 1, 4, 7, 2, 1, 2, 2, 8, 5, 1, 1, 3, 2, 4, 1, 5, 1, 2, 9, 1, 1, 1, 2, 1, 3, 3, 6, 1, 6, 2, 2, 2, 3, 1, 1, 4, 7, 10, 1, 2, 3, 11, 1, 3, 1, 1, 1, 1, 1, 4, 2, 5

Offset: 1

Gus Wiseman, Dec 27 2022

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.

			Triangle begins:
   1:   .
   2:   .
   3:   .
   4:   1
   5:   .
   6:   2
   7:   .
   8:  1 1
   9:   1
  10:   3
  11:   .
  12:  1 2
  13:   .
  14:   4
  15:   2
  16: 1 1 1
For example, the prime indices of 315 are (2,2,3,4), so row 315 is (2,3,4) - 2 + 1 = (1,2,3).

Row lengths are A001222(n) - 1.
Indices of empty rows are A008578.
Even bisection is A112798.
Heinz numbers of rows are A246277.
An opposite version is A358172, Heinz numbers A358195.
Row sums are A359358(n) + A001222(n) - 1.
A112798 list prime indices, sum A056239.
A243055 subtracts the least prime index from the greatest.
Cf. A055396, A124010, A241916, A253565, A325351, A325352, A326844, A355534, A355536, A358137.

primeMS[n_]:=If[n==1,{},Flatten[Cases[FactorInteger[n],{p_,k_}:>Table[PrimePi[p],{k}]]]];
Table[If[n==1,{},1-First[primeMS[n]]+Rest[primeMS[n]]],{n,100}]

A358172 Triangle read by rows: if n has weakly increasing prime indices (a,b,...,y,z) then row n is (z-a+1, z-b+1, ..., z-y+1).

Original entry on oeis.org

1, 2, 1, 1, 1, 3, 2, 2, 4, 2, 1, 1, 1, 2, 1, 3, 3, 3, 5, 2, 2, 2, 1, 6, 1, 1, 4, 4, 3, 2, 1, 1, 1, 1, 4, 7, 2, 2, 2, 1, 8, 5, 3, 3, 3, 4, 3, 5, 5, 2, 2, 9, 2, 2, 2, 2, 1, 3, 1, 6, 6, 6, 2, 1, 1, 3, 4, 4, 4, 7, 10, 3, 3, 2, 11, 3, 3, 1, 1, 1, 1, 1, 4, 5, 4

Offset: 1

Gus Wiseman, Dec 20 2022

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.

			Triangle begins:
   1:   .
   2:   .
   3:   .
   4:   1
   5:   .
   6:   2
   7:   .
   8:  1 1
   9:   1
  10:   3
  11:   .
  12:  2 2
  13:   .
  14:   4
  15:   2
  16: 1 1 1
  17:   .
  18:  2 1
  19:   .
  20:  3 3
For example, the prime indices of 900 are (1,1,2,2,3,3), so row 900 is 3 - (1,1,2,2,3) + 1 = (3,3,2,2,1).

Row lengths are A001222(n) - 1.
Indices of empty rows are A008578.
Even-indexed rows have sums A243503.
Row sums are A326844(n) + A001222(n) - 1.
An opposite version is A356958, Heinz numbers A246277.
Heinz numbers of the rows are A358195, even bisection A241916.
A112798 list prime indices, sum A056239.
A243055 subtracts the least prime index from the greatest.
Cf. A055396, A124010, A253565, A325351, A325352, A355534, A355536, A358137.

primeMS[n_]:=If[n==1,{},Flatten[Cases[FactorInteger[n],{p_,k_}:>Table[PrimePi[p],{k}]]]];
Table[If[n==1,{},1+Last[primeMS[n]]-Most[primeMS[n]]],{n,100}]

A251724 a(1) = 2, and for n>1: a(n) = prime(A251719(n)) * prime(A251719(n) + n - 2), where prime(n) gives the n-th prime.

Original entry on oeis.org

2, 4, 6, 21, 65, 85, 95, 115, 217, 259, 287, 301, 329, 649, 671, 737, 781, 803, 869, 913, 979, 1067, 1111, 1133, 1177, 1199, 1243, 1703, 1781, 1807, 1937, 1963, 2041, 2119, 2171, 3043, 3077, 3247, 3281, 3349, 3383, 3587, 3791, 3859, 3893, 3961, 4063, 4097, 4267, 4369, 4471, 4573, 4607, 4709, 4777, 4811, 5833, 5909, 5947, 6023, 6289, 6403, 6593, 6631, 6707, 6821, 8579

Offset: 1

Antti Karttunen, Dec 15 2014

nonn

For n >= 2: a(n) = the first "settled semiprime" in the column n of the sieve of Eratosthenes: a(n) = A083221(A251719(n), n).
The "settling of semiprimes" here means that from that semiprime onward, all the other terms in the same column n of a square array A083221 (which is constructed from the sieve of Eratosthenes) are also semiprimes, obtained by successive iterations of A003961 starting from the semiprime here given as a(n). Cf. comments in A251728 which contains all such semiprimes. The "unsettled" semiprimes are in its complement A138511.
Here we assume that A054272(n), the number of primes in interval [prime(n), prime(n)^2], is nondecreasing (implied for example if Legendre's or Brocard's conjecture is true).

Antti Karttunen, Table of n, a(n) for n = 1..10351
Wikipedia, Brocard's Conjecture

After initial 2, a subsequence of A251728 and A001358.

Cf. A000040, A003961, A054272, A055396, A078898, A083221, A138511, A243055, A251719.

a(1) = 2; and for n >= 2: a(n) = A000040(A251719(n)) * A000040(A251719(n) + n - 2).
a(n) = A083221(A251719(n), n).
Other identities implied by the definition. For all n >= 1:
A078898(a(n)) = n.
A055396(a(n)) = A251719(n).
For all n >= 2:
A243055(a(n)) = n-2.

A277314 Number of nonzero coefficients in Stern polynomial B(n,t).

Original entry on oeis.org

0, 1, 1, 2, 1, 2, 2, 3, 1, 3, 2, 3, 2, 3, 3, 4, 1, 4, 3, 3, 2, 3, 3, 4, 2, 4, 3, 4, 3, 4, 4, 5, 1, 5, 4, 4, 3, 4, 3, 4, 2, 4, 3, 4, 3, 4, 4, 5, 2, 5, 4, 4, 3, 4, 4, 5, 3, 5, 4, 5, 4, 5, 5, 6, 1, 6, 5, 5, 4, 5, 4, 5, 3, 5, 4, 4, 3, 4, 4, 5, 2, 5, 4, 4, 3, 4, 4, 5, 3, 5, 4, 5, 4, 5, 5, 6, 2, 6, 5, 5, 4, 5, 4, 5, 3, 5, 4, 5, 4, 5, 5, 6, 3, 6, 5, 5, 4, 5, 5, 6, 4

Offset: 0

Antti Karttunen, Oct 10 2016

nonn

a(n) is the number of nonzero terms on row n of A125184.

Antti Karttunen, Table of n, a(n) for n = 0..8192

Cf. A001221, A002487, A069010, A073491, A125184, A243055, A260443, A277020.

(define (A277314 n) (A001221 (A260443 n)))
;; Or as a standalone program:
(define (A277314 n) (length (filter positive? (A260443as_coeff_list n))))
(definec (A260443as_coeff_list n) (cond ((zero? n) (list)) ((= 1 n) (list 1)) ((even? n) (cons 0 (A260443as_coeff_list (/ n 2)))) (else (add_two_lists (A260443as_coeff_list (/ (- n 1) 2)) (A260443as_coeff_list (/ (+ n 1) 2))))))
(define (add_two_lists nums1 nums2) (let ((len1 (length nums1)) (len2 (length nums2))) (cond ((< len1 len2) (add_two_lists nums2 nums1)) (else (map + nums1 (append nums2 (make-list (- len1 len2) 0)))))))

a(n) = A001221(A260443(n)).
a(n) = A069010(A277020(n)).
a(n) = 1 + A243055(A260443(n)). [Because each term of A260443 is in A073491.]
a(2n) = a(n).
For all n >= 0 , a(n) <= A002487(n).

A286455 Compound filter (smallest prime dividing n & prime signature of conjugated prime factorization): a(n) = P(A055396(n), A286621(n)), where P(n,k) is sequence A000027 used as a pairing function.

Original entry on oeis.org

0, 2, 8, 2, 18, 11, 40, 2, 8, 22, 71, 11, 97, 46, 30, 2, 143, 11, 179, 22, 93, 92, 262, 11, 18, 121, 8, 46, 335, 154, 417, 2, 212, 211, 69, 11, 540, 254, 302, 22, 679, 326, 794, 92, 30, 379, 918, 11, 40, 22, 467, 121, 1051, 11, 234, 46, 530, 529, 1242, 154, 1344, 631, 93, 2, 744, 704, 1615, 211, 822, 326, 1790, 11, 1912, 904, 30, 254, 140, 947, 2167, 22, 8

Offset: 1

Antti Karttunen, May 14 2017

nonn

Note that as the other component of a(n) we use A286621 instead of A278221, because of latter sequence's unwieldy large terms.
For all i, j: a(i) = a(j) => A243055(i) = A243055(j).
For all i, j: a(i) = a(j) => A286470(i) = A286470(j).

Antti Karttunen, Table of n, a(n) for n = 1..10000
Eric Weisstein's World of Mathematics, Pairing Function

Cf. A000027, A055396, A122111, A243055, A278221, A285334, A286621, A286454, A286456.

(define (A286455 n) (* (/ 1 2) (+ (expt (+ (A055396 n) (A286621 n)) 2) (- (A055396 n)) (- (* 3 (A286621 n))) 2)))

a(n) = (1/2)*(2 + ((A055396(n)+A286621(n))^2) - A055396(n) - 3*A286621(n)).

cp's OEIS Frontend

A372589 Numbers k > 1 such that (greatest binary index of k) + (greatest prime index of k) is even.

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A325133 Heinz number of the integer partition obtained by removing the inner lining, or, equivalently, the largest hook, of the integer partition with Heinz number n.

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A372439 Numbers k such that the least binary index of k plus the least prime index of k is odd.

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A372440 Numbers k such that the least binary index of k plus the least prime index of k is even.

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A355532 Maximal augmented difference between adjacent reversed prime indices of n; a(1) = 0.

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A356958 Triangle read by rows: if n has weakly increasing prime indices (a,b,...,y,z) then row n is (b-a+1, ..., y-a+1, z-a+1).

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A358172 Triangle read by rows: if n has weakly increasing prime indices (a,b,...,y,z) then row n is (z-a+1, z-b+1, ..., z-y+1).

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A251724 a(1) = 2, and for n>1: a(n) = prime(A251719(n)) * prime(A251719(n) + n - 2), where prime(n) gives the n-th prime.

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A277314 Number of nonzero coefficients in Stern polynomial B(n,t).

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