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Numbers which have nonpositive entries in the difference table of their divisors (complement of A273130).
+20
1
6, 10, 12, 14, 15, 18, 20, 22, 24, 26, 28, 30, 34, 35, 36, 38, 40, 42, 44, 45, 46, 48, 50, 52, 54, 56, 58, 60, 62, 63, 66, 68, 70, 72, 74, 75, 76, 77, 78, 80, 82, 84, 86, 88, 90, 91, 92, 94, 96, 98, 99, 100, 102, 104, 105, 106, 108, 110, 112, 114, 116, 117
COMMENTS
Primorial numbers ( A002110) greater than 2 are in this sequence.
EXAMPLE
30 is in this sequence because the difference table of the divisors of 30 is:
[1, 2, 3, 5, 6, 10, 15, 30]
[1, 1, 2, 1, 4, 5, 15]
[0, 1, -1, 3, 1, 10]
[1, -2, 4, -2, 9]
[-3, 6, -6, 11]
[9, -12, 17]
[-21, 29]
[50]
PROG
(Sage)
def nsf(z):
D = divisors(z)
T = matrix(ZZ, len(D))
for m, d in enumerate(D):
T[0, m] = d
for k in range(m-1, -1, -1) :
T[m-k, k] = T[m-k-1, k+1] - T[m-k-1, k]
if T[m-k, k] <= 0: return True
return False
print([n for n in range(1, 100) if nsf(n)])
Prime power-like integers.
+10
4
2, 3, 4, 5, 7, 8, 9, 11, 13, 16, 17, 19, 21, 23, 25, 27, 29, 31, 32, 33, 37, 39, 41, 43, 47, 49, 53, 59, 61, 64, 65, 67, 71, 73, 79, 81, 83, 85, 89, 95, 97, 101, 103, 107, 109, 113, 115, 121, 125, 127, 128, 131, 133, 137, 139, 145, 149, 151, 155, 157, 161, 163, 167, 169
COMMENTS
Let DTD(n) denote the difference table of the divisors of n. The DTDs of prime powers (in the sense of A246655) have only positive entries and the rows and columns of their DTD are nondecreasing. We define an integer n>0 and not the unity to be prime power-like if and only if DTD(n) has only positive entries and nondecreasing rows and columns (read from left to right and from top to bottom).
Integers which have a positive but not monotone DTD are listed in A273199. Integers with a positive DTD are listed in A273130.
EXAMPLE
125 is in this sequence because it is a prime power and has the DTD:
[ 1 5 25 125]
[ 4 20 100]
[ 16 80]
[ 64]
161 is in this sequence because the DTD of 161 has only positive entries and nondecreasing rows and columns:
[ 1 7 23 161]
[ 6 16 138]
[ 10 122]
[ 112]
MATHEMATICA
pplikeQ[n_] := Module[{T, DTD, DTD2}, If[n == 1, Return[False]]; T = Divisors[n]; DTD = Table[Differences[T, k], {k, 0, Length[T] - 1}]; If[AnyTrue[Flatten[DTD], NonPositive], Return[False]]; DTD2 = Transpose[PadRight[#, Length[T], Infinity]& /@ DTD]; AllTrue[DTD, OrderedQ] && AllTrue[DTD2, OrderedQ]];
PROG
(Sage)
def is_prime_power_like(n):
if n == 1: return False
D = divisors(n)
T = matrix(ZZ, len(D))
for m, d in enumerate(D):
T[0, m] = d
for k in range(m-1, -1, -1) :
T[m-k, k] = T[m-k-1, k+1] - T[m-k-1, k]
if T[m-k, k] <= 0: return False
non_decreasing = lambda L: all(x<=y for x, y in zip(L, L[1:]))
b = True
for k in range(len(D)-1):
b &= non_decreasing(T.row(k)[:len(D)-k])
b &= non_decreasing(T.column(k)[:len(D)-k])
if not b: return False
return b
[n for n in range(1, 170) if is_prime_power_like(n)]
Integers which are prime power-like but not prime powers.
+10
3
21, 33, 39, 65, 85, 95, 115, 133, 145, 155, 161, 185, 203, 205, 215, 217, 235, 259, 261, 265, 279, 287, 295, 301, 305, 329, 335, 341, 355, 365, 371, 395, 407, 413, 415, 427, 445, 451, 469, 473, 481, 485, 497
COMMENTS
For an integer n>0 and not the unity we define DTD(n) to be the difference table of the divisors of n. We say that DTD(n) is positive if all entries in the table are positive and we call DTD(n) monotone if all rows and all columns of the table are nondecreasing (reading from left to right and from top to bottom).
We define an integer n to be prime power-like if and only if DTD(n) is positive and monotone. All prime powers (in the sense of A246655 (but not in the sense of A000961)) are prime power-like integers. Sequence A273200 provides the prime power-like integers. This sequence ( A273201) lists the integers which are prime power-like but not prime powers.
EXAMPLE
95 is in this sequence because the DTD of 95 has positive entries and nondecreasing rows and columns:
[ 1 5 19 95]
[ 4 14 76]
[10 62]
[52]
MATHEMATICA
pplikeQ[n_] := Module[{T, DTD, DTD2}, If[n == 1 || PrimePowerQ[n], Return[False]]; T = Divisors[n]; DTD = Table[Differences[T, k], {k, 0, Length[T]-1}]; If[AnyTrue[Flatten[DTD], NonPositive], Return[False]]; DTD2 = Transpose[PadRight[#, Length[T], Infinity]& /@ DTD]; AllTrue[DTD, OrderedQ] && AllTrue[DTD2, OrderedQ]];
PROG
(Sage) # uses[is_prime_power_like from A273200] return not is_prime_power(n) and is_prime_power_like(n)
print(list(filter(is_ A273201, range(1, 500))))
Integers which have a positive but not monotone difference table of their divisors.
+10
2
51, 55, 57, 69, 87, 93, 111, 119, 123, 129, 141, 159, 177, 183, 201, 207, 213, 219, 237, 249, 253, 267, 275, 291, 303, 309, 319, 321, 327, 333, 339, 369, 377, 381, 393, 403, 411, 417, 447, 453, 471, 489, 501, 519, 537, 543, 573, 579, 591, 597
COMMENTS
For an integer n>0 and not the unity we define DTD(n) to be the difference table of the divisors of n. We say that DTD(n) is positive if all entries in the table are positive and we call DTD(n) monotone if all rows and all columns of the table are nondecreasing (reading from left to right and from top to bottom).
EXAMPLE
159 is in this sequence because the DTD of 159 has only positive entries but not all columns are nondecreasing:
[ 1 3 53 159]
[ 2 50 106]
[ 48 56]
[ 8]
PROG
(Sage)
D = divisors(n)
T = matrix(ZZ, len(D))
for (m, d) in enumerate(D):
T[0, m] = d
for k in range(m-1, -1, -1) :
T[m-k, k] = T[m-k-1, k+1] - T[m-k-1, k]
if T[m-k, k] <= 0: return False
non_decreasing = lambda L: all(x<=y for x, y in zip(L, L[1:]))
b = True
for k in range(0, len(D)-1):
b &= non_decreasing(T.row(k)[:len(D)-k])
b &= non_decreasing(T.column(k)[:len(D)-k])
if not b: return True
return False
print([n for n in range(1, 600) if is_ A273199(n)])
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