a = [1, 2, 5, 6, 100, 101, 1000, 1001, 100000, 100002, 100004]
d = {}
 
for i in range(len(a)):
    for j in range(i + 1, len(a)):
        k = a[j] - a[i]
        if k not in d: d[k] = [1, i]
        else: d[k][0] += 1
 
# gives us dictionary, key is a difference between elements,
# first element of list is count of pairs having this diff, second element is
# starting point
# {
#    1: [2, 1],
#    2: [1, 2],
#    3: [3, 0],
#    4: [3, 0],
#    5: [1, 3],
#    6: [1, 0],
#    7: [2, 0],
#    8: [1, 1],
#    11: [1, 0]
# }
 
x = max(d.iteritems(), key=lambda x: x[1][0])
diff = x[0]
j = x[1][1]
result = [a[j]]
for i in range(j, len(a)):
    if a[i] - a[j] == diff:
        result.append(a[i])
        j = i
 
print result

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