An amenable number is a positive integer for which there exists a multiset of as many integers as the original number that both add up to the original number and when multiplied together give the original number. To put it algebraically, for a positive integer n, there is a multiset of n integers {a1, ..., an}, for which the equalities
hold. Negative numbers are allowed in the multiset. For example, 5 is amenable since 5 = 1 + (-1) + 1 + (-1) + 5. All and only those numbers congruent to 0 or 1 (mod 4), except 4, are amenable. (Tamvakis & Lossers 1998)
The first few amenable numbers are: 1, 5, 8, 9, 12, 13 ... OEIS: A100832
A solution for integers of the form n = 4k + 1 could be given by a set of 2k (+1)s and 2k (-1)s and n itself. (This generalizes the example of 5 given above.)
Although not obvious from the definition, the set of amenable numbers is closed under multiplication (the product of two amenable numbers is an amenable number).
All composite numbers would be amenable if the multiset was allowed to be of any length, because, even if other solutions are available, one can always obtain a solution by taking the prime factorization (expressed with repeated factors rather than exponents) and add as many 1s as necessary to add up to n. The product of this set of integers will yield n no matter how many 1s there are in the set. Furthermore, still under this assumption, any integer n would be amenable. Consider the inelegant solution for n of {1, -1, 1, -1, n}. In the sum, the positive ones are cancelled out by the negative ones, leaving n, while in the product, the two negative ones cancel out the effect of their signs.
Amenable numbers should not be confused with amicable numbers, which are pairs of integers whose divisors add up to each other.
Well, that’s interesting to know that Psilotum nudum are known as whisk ferns. Psilotum nudum is the commoner species of the two. While the P. flaccidum is a rare species and is found in the tropical islands. Both the species are usually epiphytic in habit and grow upon tree ferns. These species may also be terrestrial and grow in humus or in the crevices of the rocks.
View the detailed Guide of Psilotum nudum: Detailed Study Of Psilotum Nudum (Whisk Fern), Classification, Anatomy, Reproduction
Well, that’s interesting to know that Psilotum nudum are known as whisk ferns. Psilotum nudum is the commoner species of the two. While the P. flaccidum is a rare species and is found in the tropical islands. Both the species are usually epiphytic in habit and grow upon tree ferns. These species may also be terrestrial and grow in humus or in the crevices of the rocks.
View the detailed Guide of Psilotum nudum: Detailed Study Of Psilotum Nudum (Whisk Fern), Classification, Anatomy, Reproduction